In the present clinical fields, medical doctors mainly use Modified Ashworth Scale (MAS) to evaluate spasticity. However, MAS is manual and qualitative method and results depend on doctors. The purpose of this paper is to propose a method to quantitatively evaluate spasticity using a neuro-musculo-skeletal model. The proposed model consisted of the muscular activation model, variable viscoelastic model of the muscles, musculoskeletal model, muscle spindle model and alpha motoneuron model. This model could represent spasticity patient’s knee pendulum well. A parameter related with sensitivity of the muscle spindle was a factor to evaluate spasticity quantitatively.