Organisms resist various risks (such as natural enemy, diseases, competition with other species, the variance of growth rate, etc) in terms of various manners of life history strategies. Above all, it is obtained with mathematical analysis（Tuljapurker, 1990） that variance of growth rate reduces fitness. In fact, it is known that sensitivity of the fitness to each parameter has negative correlation with its variance in some species ,by using the transition matrix model（Pfister,1998）. However, there are a few papers that the models include the optimal reproductive strategy with stochasticity in size structure. Therefore, we directly derived a theoretical model, which is taken into account the optimal reproduction timing in their stochastic growth process, from typical linear demographic models (such as the transition matrix model and integration projection model (Zuidema, et.al 2010). Then, we present our analysis on the optimal reproductive timing in their life history, based on our model .