The mantis shrimp (Oratosquilla oratoria) is a benthic, megafaunal burrower and a predatory species. As an important commercial fishery species, its abundance is significantly correlated to sediment type. To understand the size at first maturity of female mantis shrimp widely distributed in Bohai Bay, the biological data, such as carapace length, kubo's standard length, total length, weight, gonadal development and ovarian weight, of female individuals were collected monthly from April to November except May in 2018 from research surveys on the board of fishing vessels. The determination of the immature and mature stages was performed based on the gonadal development, and then the size at first maturity was estimated using a logistic model with specific asymptote or variable asymptote. The results showed that:Female mantis shrimp could develop to maturity and complete reproduction at a standard length of 7.00-7.99 cm; Female individuals with 9.00-11.99 cm standard length made a great contribution to reproduction in quantities; There was a bias for estimates of the size at first sexual maturity between different sexual maturities judged by gonadal development, as well as models with different asymptotes; Estimates of size at first sexual maturity based on a logistic curve with variable asymptote were (9.42 ±0.02) cm, (10.44 ±0.04) cm and (12.03 ±0.17) cm respectively by using gonad stage of Ⅱ, Ⅲ and Ⅳ as criteria of mature for female mantis shrimp in Bohai Bay. And while the asymptote was assigned to 1 in the logistic model, the estimates of the size at first sexual maturity were (9.44 ±0.02) cm, (10.50 ±0.04) cm and (11.74 ±0.08) cm, respectively. This study enhanced our understanding of the relationships between the proportion of maturity and standard length of female mantis shrimp. While female mantis shrimp with gonadal development stages Ⅰ were treated as immature and individuals with gonadal development stages Ⅱ and above as mature, the size at first sexual maturity could be robustly estimated by using a logistic curve with variable asymptote through describing the relationship between the proportion of maturity and standard length. Based on these findings, it was suggested that the regional fishery should operate with a minimum legal size of 9.42 cm for mantis shrimp.