Modifying LIBS += -L$${DESTDIR} -lprojectsub1 in projectsub1.pri on the line of LIBS += -L$$OUT_PWD/../projectsub1/debug/ -lprojectsub1 in projectgui.pro (this line was generated by "Add Library" menu option in QtCreator) seems to work. It was looking for dll in /debug/ subfolder in {DESTDIR}.