Biodiesel is an environmentally friendly alternative fuel consisting of the alkyl monoesters of fatty acids. Unlike the hydrocarbon chain structure in diesel, the degradation of alkyl monoesters of fatty acids in biodiesel may deteriorate non-metallic parts in diesel engine especially those made from elastomer, polymer, plastic and rubber. In this study, elastomer parts from fuel system of typical diesel engine in Thailand were investigated. All samples were immersed in B100, diesel and B100 added with methanol which made some properties out of the recommended range suggested by recommended standard in order to compare the effect of the fuels on the materials strength and dimension changes. The elastomer samples were prepared using SAE (Society of Automotive Engineering) and ASTM (American Society of Testing and Materials International) standards and tested at an elevated temperature of 100oC for 22, 60, 670 and 1006 hours. The following properties were measured and recorded before and after immersion: appearance, mass, volume, dimension, hardness and weight after dry-out. All of the samples showed degree of swell after immersion in all fuels. Change in volume of materials was increased and it often accompanied by a decrease in hardness or softening. At the end of the immersion, all materials showed a small change in dimension. The hardness measurement after immersion in B100 and B100 with methanol added exhibited better hardness properties on some specific parts when compare with diesel fuel.