The plot of detonation velocity vs. inverse radius curves and critical diameters (d_er) were obtained for PETN single crystals shocked parallel[001] and [110] crystal planes, d_er was found dependent on the detonation wave orientation, being 11～14 mm for the [110] and 5～7 mm for the [001]planes. New results were compared with the previous data for PETN single crystal d_er. and J. Dick’s shock initiation experiments.