A process for preparing a nitride based semiconductor device in accordance with the present invention comprises growing a high temperature AlN single crystal layer on a substrate; growing a first GaN layer on the high temperature AlN single crystal layer in a first V/III ratio, under a first pressure of 300 Torr or more, such that the predominant direction of growth is the lateral direction; and growing a second GaN layer on the first GaN layer in a second V/III ratio lower than the first V/III ratio, under a second pressure lower than the first pressure such that the predominant direction of growth is the lateral direction.