When a femtosecond laser pulse is focused tightly inside a LiF single crystal, cracks are generated in the < 100 > directions from the photoexcited region. Previously, we found that cracks of different lengths are formed by simultaneous fs laser irradiation at multiple spots. To elucidate the mechanism of modulation in crack lengths, we observed the transient stress distributions after simultaneous fs laser irradiation at multiple spots inside a LiF single crystal. First, we found that stress amplitude can be doubled by the interference of fs laser induced stress waves. Next, we observed the dynamics of crack formation as well as the transient birefringence distribution. In the case in which one crack became shorter than other cracks, the observation of the crack dynamics showed that the compressive stress by a constrictive interference of stress waves at a crack tip prevented the crack from propagating further. However, In the case in which the elongation of one crack occurred, we could not find any relationship between the elongation of a crack and the interference of stress waves by the observed stress distribution. Based on the time-resolved observation, we discussed the mechanism of the modulation of the laser induced cracks by interference of stress waves.