Rail support modulus is an important factor in safety of railway track. This parameter is defined as support force imposing on rail length unit per rail unit displacement in vertical direction. Rail support modulus is important because it affects track performance and maintenance requirements. Both low and large modulus is undesirable. Low modulus of the rail support has been shown to cause differential settlement that then increases maintenance needs. On the other hand, higher value of rail support causes axle load to be distributed on fewer sleepers and therefore received dynamic loads for any sleeper and rail-sleeper fastening will be increased. Sandy desert areas are critical regions about the contamination of ballast. In these areas, flowing sand grains influence between ballast aggregates and increase the stiffness of ballast layer and the rail support modulus. In this paper, the results of a field investigation about the effect of ballast contamination on the values of the rail support modulus in sandy desert areas are presented.