@Tee_ normally a pro file results in a separate target (executable or library). Even if you "combine" them with a subdirs project. So your AppUi project must link against the AppLib

@rtavakko I'm so pleased to hear the working solution! Thanks for sharing the good news :)

Hi, What are these "minor things" ?

After few days searching on Google. I found the answer. The problem is the order you Link to Static Library. Dependency of my Projects is: Presentation << Utilities << DynamicObjectCore in Presentation.pro, I'm link to Static Link Library in this order # Link to DynamicObjectCore ... # Link to Utilities ... I thought that it will compile DynamicObjectCore first, then compile Utilities. But it wan't, and cause the compile error. "undefined reference to..." When i change the Link Order # Link to Utilities ... # Link to DynamicObjectCore ... And it work.