This is the first study to genetically analyze the relative growth of multiple partial body sizes in relation to the entire body size in rainbow trout, Oncorhynchus mykiss. The experimental groups were the first generation hybrid group (F1) and the second generation hybrid group (F 2) obtained from the Bohai Cold Water Fish Experimental Station of the Heilongjiang River Fishery Research Institute, Chinese Academy of Fishery Sciences. By stepwise regression analysis, we optimized the phenotypic joint static/ontogenetic allometry scaling model to select the significant partial allometry scalings of multiple partial body sizes to entire body size. Two random regression models were constructed to genetically analyze multiple static/ontogenetic allometries. One model embeds the best joint static allometry model in to the fi xed and additive genetic effects of a simple animal model. The other model considers not only the fixed and additive genetic eff ects on the best joint ontogenetic allometry model, but also the time dependence of permanent environmental effects in the repeated records animal model. The genetic parameter estimation of multiple allometries can be implemented with restricted maximum likelihood for a random regression model. Application of the models proposed here is illustrated to genetically analyze jo int sta tic allometry scalings of multiple body shape traits to body weight and ontogenetic allometry scaling of body lengths to body weights repeatedly observed at different growth times in rainbow trout. The results showed that body length has the largest significant allometric association with body weight, and the order in genetic variances for allometry scalings is li kely to be consistent with that of phenotypic partial allometry scalings. The largest genetic correlation for allometry scaling was found to be −0.8675 between body width and dorsal-fin base length, followed by −0.6194 between body length and body depth, w hile −0.0217 was the minimum value. We estimated additive genetic variance for ontogenetic allometry of the body length to weight ratio as 0.2929. According to this study, the allometries of the morphological traits to body weights not only quantify differences in relative growth bet ween the morphological traits to body weights, but also measure the relative genetic gains of the morphological traits to the body weights by genetic analysis. Thus, the genetic parameter estimation for allometries may be useful for guidance in synchronously improving morphological traits and body weights in rainbow trout. This will help genetic manipulation of body shape as well as genetic improvement of body weight.