In this paper, single crystals, around 8mm in diameter, of MoSi2 and NbSi2 have been grown by optical heating floating zone method. X-ray analysis confirmed that the as-grown ingots were single phase and single crystalline material. Oxidation behavior of the poly-crystalline and single crystalline MoSi2 and NbSi2 were characterized by measuring their weight changes as a function of exposure time. For arc-melted poly-crystalline samples, MoSi2 and NbSi2 fully turned into white powders after 160 and 3hrs exposure at 773K and 1023K respectively, which is known as the “pesting” phenomenon. As a comparison, no pesting was found in the dense spark plasma sintered (SPS) poly-crystalline samples and single crystals. The weight change of single crystals during exposure is found to be much lower than that of the SPS sample, indicating grain boundary plays an important role in the low temperature oxidation behavior of these two silicides.