To explore the effect of KK-42 on the molt cycle of juvenile Macrobrachium nipponense and its possible mechanism, the juvenile prawns(1.2-2.0 cm long) were collected from our aquaculture ground, and acclimated at(26±1)℃ in running-water tanks in the laboratory and fed twice daily for 1 week before the experiments. The prawns were administered with KK-42 at a concentration of 0(control group) or 1.95×10^-4 mol/L(treatment group) for 1 min, respectively. A portion of them were used to measure the growth rate and molt cycle of juvenile prawns; and the rest, being at intermolt stage(C) and premolt stage(D), were used to analyze quantitatively cuticular chitinase1 gene(Mnchi-1) mRNA level and the chitinase activity. The results showed that the average weight of treatment group was significantly higher than that of the control one, and its growth rate increased significantly in the first two weeks after KK-42 treatment. The duration of molting period tended to prolong following the growth of juvenile prawns, and that was shortened by KK-42 treatment at the first two cycles among four consecutively-determined molt cycles, from(8.70±1.07),(9.81±0.43)d/molt cycle to(6.93±0.97),(8.11±1.20)d/molt cycle, respectively. The culticular Mnchi-1 mRNA level as well as the chitinase activity in the stage C increased by more than 10 and 2 times, respectively, at 3, 6 and 9 h after KK-42 treatment, but those in the stage D, only had a modest rise at 12 h. The results display that KK-42 can importantly up-regulate the culticular Mnchi-1 gene expression and chitinase activity in stage C and D, which may result in the old cuticle degradation to be triggered in stage C, suggesting one of the molecular mechanisms of KK-42 of shortening molt cycle and promoting growth effect for juvenile M. nipponense.