The fluctuation model of passively mode-locked lasers has been improved. A com-puter simulation of the evolution of light pulses was performed. The simulation started at the point when the gain coefficient had just increased to the threshold value. The starting average intensity was calculated in accordance with the spontaneous emission The rate of pumping, the life-time of the upper laser level and the ratio of the area of beam section at the gain medium to that at the saturable absorber were taken into ac-count. The calculation has been done for Nd-glass and Nd: YAG lasers.The simulation calculation led to the following conclusions: (1) It is impossible to obtain complete mode locking with a confidence over 93% when the pumping rate exceeds the threshold value by 15 to 20%. (2) The optimum ratio of the area of the beam sections is from 1/2 to 1 for Nd-glass lasers, and larger than 2 for Nd: YAG lasers. (3) To obtain complete mode locking the transmission of the saturable dye must be within a certain range (depending on the loss coefficient and the ratio of the beam sections). The transmission of the dye for Nd-glass lasers should be higher than that for Nd: YAG lasers, if the same dye is employed.Finally, the shortcomings of Glenn's calculation are pointed out.