The purpose of this study was to investigate the effects of clinical characteristics of chronic stroke patients on physiological cost index (PCI) during walking. Fourteen stroke patients participated in this study. To investigate the clinical characteristics, Fugl-Meyer score (FMS), gait velocity (GV), muscle strength of the knee extensor, modified Ashworth scale (MAS) of ankle plantar flexor, devices, and gait patterns during walking were measured and analyzed. The results were as follows: Firstly, use of devices and high MAS of the ankle plantar flexor significantly increased PCI. Secondly, PCI was significantly correlated with the FMS and MAS of the ankle plantar flexor. In conclusion, inhibition of spasticity of the ankle plantar flexor is considered to reduce PCI during walking for chronic stroke patients.