A noninvasive method of diagnosing early knee osteoarthritis is propos ed.First,a simplified optical model of knee joint issue irradiated by near infrared light i s developed and the propagation of photons in three dimensional (3D) knee joint issue is simulated b y Monte Carlo (MC) method.Then,light distributions on the surface of knee joint issues are recorded when absorpti on or scattering coefficient is altered.The results show that there are useless photons,which di rectly migrate out of knee issues,and important photons carrying optical parameter information,whic h collide with bone issue and then migrate out of knee issues.It also shows that changes of ab sorbance and scattering coefficients will make light distribution different.Therefore,early k nee osteoarthritis can be diagnosed by the changes of light distribution.