This paper addresses a plantar H-slit force sensor for humanoid robots to detect the reaction force. The zero moment point (ZMP) is calculated from the detected reaction force balance. The H-slit force sensors are structuralized on a plantar frame. Since a H-slit beam unidirectionally deforms, only the normal force from the ground is selectively detected without being affected from the friction force between the sole and ground. The plantar H-slit force sensors are developed for a humanoid robot. A model is proposed to simulate the sensor output. The simulated outputs are compared with results of the finite element analysis (FEA) and the experimentation results.