[Objectives] Mesocentrotus nudus is a key species in maintaining the stability of the kelp ecosystem and a valuable economic sea urchin species in northern China. However, little researches focus on its population ecology, and the factors impacting the population dynamics are still not clear. Gaetice depressus, Hemigrapsus sanguineus, Portunus trituberculatus and Charybdis japonica are four common crab species off the coast of China and are potential predators of M. nudus. In order to explore the predation strategies and predation intensities of four crab species and the predation selectivities of C. japonica on different sizes of M. nudus, three different groups of body sizes of M. nudus individuals, e.g., small sized (12﹣15 mm), medium sized (19﹣22 mm) and large sized (30﹣33 mm in diameter) were selected as the research objects. [Methods] Two sets of experiments were conducted in the laboratory. In the predation strategies and predation intensities experiment, each species of crabs was divided into two groups separately containing small and medium sized sea urchins, while the predation selective experiment contains a C. japonica and three sizes of urchins in a single experimental group. The data was analyzed using Excel and Spss16.0, and the significance was inspected using one-way ANOVA and Kruskal-Wallis in both sets of experiment, and Ivlev indexes were calculated to examine the predation selectivity of C. japonica on small and medium sized sea urchins. [Results] The results showed that four crab species could prey on the small sized sea urchin individuals, of which, the P. trituberculatus and C. japonica showed higher predation intensity on sea urchins (8.67 ± 1.53 and 9.33 ± 1.15, respectively, Table 2); whereas only P. trituberculatus and C. japonica consumed the medium sized sea urchins (Table 2). As for the predation strategies, four crab species presented diverse kinds of strategies on different sizes of sea urchins but showed similar predation behavior when encountering difficulties of crushing the urchins (Fig. 1). The predation and selection pattern of C. japonica belong to a kind of passive selection, and the selectivity was closely related to the size of its cheliped. Crabs with smaller cheliped had relatively high selective feeding index on small sized sea urchins (Fig. 2), while individuals with bigger cheliped showed no significant differencein predation rate for small and medium size sea urchins (one-way ANOVA, P > 0.05) (Fig. 2). [Conclusion] This study reveals that G. depressus, H. sanguineus, P. trituberculatus and C. japonica are all potential predator of sea urchin M. nudus. Moreover, they can adapt their hunting strategies according to the preys with different body size, predation capacity is a key factor to the prey selectivity of C. japonica.