///My globals.h file

#ifndef GLOBALS_H
#define GLOBALS_H

#define GEOMETRY_X 0
#define GEOMETRY_Y 0
#define GEOMETRY_HEIGHT 320
#define GEOMETRY_WIDTH 240

/////My createnew.h file

#ifndef CREATENEW_H
#define CREATENEW_H

#include <QtGui>
#include <QWidget>
#include <QMainWindow>
class CreateNew : public QMainWindow
{
Q_OBJECT

public:
explicit CreateNew(QWidget *parent = 0);

private:
QVBoxLayout *mainLayout;
QVBoxLayout *midLayout;
QHBoxLayout *bottomLayout;
QLabel *title;

QPushButton *btnSave;
QPushButton *btnClose;

signals:

private slots:

void on_btnSave_clicked();
void on_btnClose_clicked();

protected:
void keyPressEvent(QKeyEvent *event);
};

#endif // CREATENEW_H

/////My createnew.cpp file

#include "createnew.h"
#include "globals.h"

CreateNew::CreateNew(QWidget *parent) :
QMainWindow(parent,Qt::FramelessWindowHint)
{
setGeometry(GEOMETRY_X, GEOMETRY_Y+25, GEOMETRY_WIDTH, GEOMETRY_HEIGHT);
QWidget *win = new QWidget(this);
setCentralWidget(win);

mainLayout=new QVBoxLayout(win);
midLayout=new QVBoxLayout();


midLayout->setGeometry(QRect(0,50,180,260));
title=new QLabel("Test Form");
midLayout->addWidget(title);

QString str;
for(int i=0; i<20;i++){
    QCheckBox *qcb=new QCheckBox("Item # :"+str.setNum(i+1));
    qcb->setStyleSheet("width:80px");
    midLayout->addWidget(qcb);
}
mainLayout->addLayout(midLayout);

QScrollArea *scrollArea=new QScrollArea(this);
scrollArea->setWidget(mainLayout->widget());
scrollArea->setStyleSheet("QScrollBar:vertical { width: 5px; }");

bottomLayout=new QHBoxLayout();
btnSave=new QPushButton("Save");
btnClose=new QPushButton("Close");
bottomLayout->addWidget(btnSave);
bottomLayout->addWidget(btnClose);
mainLayout->addLayout(bottomLayout);

connect(btnClose, SIGNAL(clicked()), this, SLOT(on_btnClose_clicked()));
connect(btnSave, SIGNAL(clicked()), this, SLOT(on_btnSave_clicked()));

}

void CreateNew::on_btnSave_clicked()
{
this->close();
}
void CreateNew::on_btnClose_clicked()
{

}

void CreateNew::keyPressEvent(QKeyEvent *event)
{
if (event->key() == Qt::Key_Escape ){
this->close();
}
}

Yes you are missing something. Your Widget (win) has no layout.
You set "win" as widget to your ScrollArea, then you set ScrollArea as CentralWidget to your MainWindow.

win->setLayout(mainLayout);

Should fix it.

I can add scrollbar to the Frameless window but cannot add scrollbar only to the Layout. Somehow I am missing the good point.

I am not using any UI

Why not?

@Rumzi said in Want to add scroll area only to Layout not entire window:

I could't find any way to add the scroll bar to mainLayout except win (Widget)

Just adding a random scrollbar to your layout is not possible. You need something like a QScrollArea and put your widget with your layout inside that area.
EDIT: Didnt realized, that you are already using a QScrollWidet :) But you still cant change the scroll + scrollbar behavior per default
What do you mean by

add the scroll bar to mainLayout except win (Widget)

Just place your title label at the top and the buttonbox below outside of the QScrollArea

@Rumzi said in Want to add scroll area only to Layout not entire window:

I want to scroll the scrollbar on mouse move/ scroll/ drug up and down.

To scroll by moving the mouse to the top / bottom of the widget, you have to implement it on your own by tracking your MouseMovement inside your widget.

EDIT_2:

@Rumzi said in Want to add scroll area only to Layout not entire window:

inheriting QMainWindow of Qt::FramelessWindowHint

Instead of setting the FramelessWindow-flag to your MainWindow, better create a frameless widget / dialog