Abstract: This paper is a comparison study of the mechanical performance of the red muscle in vivo in cruise swimming of anguilliform and carangiform fishes, based on a global modeling of the integrated swimming mechanics. Given a lateral undulatory motion of the fish body, this modeling method could ascertain the external fluid force and the inertia force, and then indirectly predict the in vivo internal active muscle force and viscoelastic force due to the passive deformation of biological tissues. The results suggest that the strength of the caudal red muscle is significantly lower than that of the anterior in an anguilliform swimmer, while in a carangiform swimmer, the strength of red muscle is roughly equal in caudal and anterior part of the body. A higher energy efficiency is observed for a carangiform swimmer as a result of the different external forces experienced in the two swimming modes. It is also found that in both swimming modes, there exists an energy transmission from the anterior to the posterior, and the distribution of the net power produced by the active muscle along the fish body is characterized by a bell-shaped curve. Generally, the positive net power is produced at each axial position.