In order to disparse the time of hatching, several batches of eggs of the tobacco cutworm, Spodoptera litura Fabricius, deposited within 4 hours were kept in constant temperature cabinets of 21.0, 22.5, 24.0, 25.5 and 27.0℃. The hatched larvae were reared on an artificial diet at 25℃ under a regime of 12L : 12D. The moulted individuals at the end of the fifth larval instar were counted at 4 hour intervals over a 24 hour period, that is, at 0, 4, 8, 12, 16 and 20 hours. The numbers of individuals of pupation and emergence were also counted at the same hours. By this method of experiment, the cycles of moulting, pupation and emergence were possible to synchronized with daily. The results of harmonic analysis showed that these daily rhythms are well described by the mathematical model of sine curve. The calculated sine curve equations are Y=2.3775-0.5242u_1+1.3423v_1 for moulting, Y=2.5771-1.1200u_1-0.3594 v_1 for pupation, and Y=1.5437-0.8993u_1-0.2650v_1 for emergence. The estimated maximum times of moulting, pupation and emergence are 7.25, 13.11 and 5.05, respectively. Hence, the estimated minimum times would be 12 hours earlier at 19.25, 1.11 and 17.05.