Study on the maturity stages of spiny lobster, Panulirus ornatus had been made using gonadosomatic index, (GSI) and the gonad structures. A total of 93 female Panulirus ornatus of the size range from 27.6 mm to 139.7 mm CL and 103 males of 45.2 - 139.0 mm CL had been used in the study. The samples for the study were collected from Lahad Datu, Sabah, Malaysia from April to November, 1998. Their characteristics for sexual dimorphism were also discussed. The results show that there were no significant difference between carapace length and body weight for male and female specimens but regression between the total length and body weight showed a significant difference between male and female specimens. It was found that the female Panulirus ornatus matures at size 107.2 mm CL while the male at 98.1 mm CL.