1-I want to know will onPostExecute executes by considering that activity is not displaying on screen anymore?
yes!
2-In case answer is yest, will it cause exceptions or not? (because of accessing UI objects which are not longer displayed on screen).
Yes! it may cause Exception because your Instance of your Activity and Views which you use in your AsyncTask are not exist anymore
This Link will help you more : AsyncTask won't stop even when the activity has destroyed