A dynamic-crack-curving criterion, which is valid under pure Mode I or combined Modes I and II loadings and which is based on either the maximum circumferential stress or minimum strain-energy-density factor at a reference distance ofr 0 from the crack tip, is verified with dynamic-photoelastic experiments. Directional stability of a Mode I crack propagation is attained when $$r_o= \frac{1}{{128\pi }}(\frac{{K_r }}{{\sigma _{ox} }})^2 V_o^2 (C,C_1 ,C_2 ) > r_c $$ wherer c =1.3 mm for Homalite-100 used in the dynamic-photoelastic experiments.