Abstract: An incompressible turbulent boundary layer separation in a two dimensional diffuser was surveyed using laser doppler velocimeter,and its time averaged velocities and turbulence quantities were obtained.The time averaged velocity profiles were compared with several empirical relationships.Prediction was made by using a quasi simultaneous viscous/inviscid interaction model and was compared with the experiment.The prediction with two empirical relationships of shape facter H 1=H 1(H) was compared.The prediction is in agreement with the experiment before the strong reversing flow.