In this paper optical properties of silk fibroin (SF) thin films obtained via spin coating method are investigated. Work concentrates on the experimental examination of influence of relative humidity (RH) in the range of 7-90% in air on the changes of SF refractive index, thickness and optical path at room temperature. Spectral ellipsometry and interference methods are used for the SF thin films optical properties measurements. The sensing structure response value, response time and recovery time are presented. The results shows that RH significantly influenced the SF thin films thickness and refractive index what should be taken into account during the designing of optical devices based on SF. It also shows that SF thin films are promising material for room temperature RH sensors.