The SnO2 nano particles were prepared by microwave technique. The particle sizes of SnO2 were confirmed by XRD and TEM analysis. The prepared nano particles have good humidity sensing property which will be adopted for analyzing the evaporation in the solar still. The humidity measurements are carried out by fixing the nano particle pellet inside the solar still at the required place. The humid variations are measured inside the solar still with respect to the water and air temperatures.