in the north Yellow Sea was estimated using Bayesian methods. The relationship was calculated for both the assumption of an additive error structure and for a multiplicative error structure. The species form factor was calculated according to estimated parameters from this study and those found in the literature. Samples were obtained from the commercial catch of trawls at Weihai Port and Shidao Port and of set nets in Shuangdao Bay in Shandong. Bayesian methods effectively estimated parameters of the weight–length relationship and their uncertainties. The deviance information criterion selected the weight–length relationship with multiplicative error structure as the best model from the candidate models. The weight–length relationships of Japanese anchovy in spring, autumn, and winter were 3.1774, and, respectively. There were significant differences among the seasonal curves of the weight–length relationship. The springtime curve was located above those of autumn and winter, while the autumnal curve was similar to that of winter. Thus, the relationship for pooled data across seasons would underestimate the weight in spring and overestimate autumn and winter weights. Parameter showed an increasing trend since 2004, where estimated values between 2004 and 2014 were distinctly larger than those prior to 2004. The weight–length curves since the late 1990s were located above those from the 1960s to the1980s, implying the mean weight for a given size fish had increased. The total length–fork length (TL–FL) relationship of Japanese anchovy in the north Yellow Sea was TL=0.0615+1.0909FL. The form factor of Japanese anchovy ranged from 0.004 73 to 0.006 60 (median=0.006 03), which could serve as an index for the characteristic shape of Japanese anchovy. Thus, this study provided basic parameters needed to evaluate Japanese anchovy stocks and to further investigate the response mechanism of Japanese anchovy to fishing pressure and environmental changes.