Thank you wysota. Your suggested solution gives me better result.

Qt Code: 
  1. void MainWindow::createLayerDockWidget() { 
  2.  
  3. ...
  4.  
  5. QWidget * layerWidget = createLayerWidget();
  6.  
  7. QDockWidget *dock = new QDockWidget(tr(" Layers "), this);
  8. dock->setBackgroundRole(QPalette::Light);
  9. dock->setAutoFillBackground(true);
  10. dock->setAllowedAreas(Qt::LeftDockWidgetArea | Qt::RightDockWidgetArea);
  11.  
  12. QScrollArea * scrollArea = new QScrollArea;
  13. dock->setWidget(scrollArea);
  14. scrollArea->setWidget(layerWidget);
  15. scrollArea->setAlignment(Qt::AlignTop);
  16. scrollArea->viewport()->setBackgroundRole(QPalette::Light);
  17. scrollArea->viewport()->setAutoFillBackground(true);
  18. scrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
  19.  
  20. addDockWidget(Qt::RightDockWidgetArea, dock);
  21.  
  22. }
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Thank you MrDeath too. Don't surprise I am new in Qt.