Shear strength of unsaturated soils is an important engineering property that is required for addressing geotechnical problems, the prediction of which remains to be a challenging task for design engineers due to the complex interaction problem. This study presents a new shear strength equation based on the micromechanical model and the disturbed state concept for unsaturated soils. The original point of this study is considering the solid contact area ratio which was neglected in most of the existing equations. Using the proposed model, the non-linear relationship between the matric suction, saturation degree, and the shear strength of unsaturated soils are described. Validation of the shear strength model was verified against the experimental data and several current models on six different types of soils. The results indicate that the proposed model has a good performance in predicting the shear strength of unsaturated soils, and generally is better than other existing models. In response to varying climatic conditions, the analytical model was then extended to consider the effect of temperature on the shear strength of unsaturated soils. The comparison between predicted and measured results was carried out on compacted silt for three different temperatures. The results show that the proposed model is capable of accurately predicting changes in unsaturated shear strength as a function of temperature.