When femtosecond laser pulses are focused inside a single crystal, anisotropic structural changes such as dislocation and cleavage occur along specific orientations. It can be interpreted that the anisotropic structural changes should be induced by transient stress after photoexcitation, such as a thermal stress and stress wave. To elucidate the mechanism of the laser induced structural changes inside crystals, we developed a novel time-resolved polarization imaging system, in which circularly polarized laser pulse was used as a probe light. The system enabled us to observe laser-induced transient stress distribution as well as the orientation after focusing fs laser pulses inside MgO and LiF single crystals. Based on the observation, we elucidated the relation between laser-induced transient stress distribution and anisotropic structural change inside the crystals. © 2012 SPIE.