Atomic order thermal nitridation of Si1-xGex(100) (x=0, 0.4, 1) by NH3 gas environment at low temperature of 400ºC has been investigated using an ultraclean low-pressure chemical vapor deposition system. After NH3 exposure (NH3 partial pressure of 550 Pa for 30 min), it is found that Si(100), Si0.6Ge0.4(100) and Ge(100) are nitrided in atomic-order. In the case of Ar purging at cooling period after the nitridation, N atom amounts on Si(100) and Ge(100) are 8.0x1014 and 5.5x1014 cm-2, and the amount on Si0.6Ge0.4(100) is 12.5x1014cm-2. In the case of H2 purging at the cooling period, it is found that slight reduction of the N atom amount on Si(100) down to 5.6x1014 cm-2(70 %) is observed, although the N atom amount on Ge(100) is significantly reduced to 1.2x1014 cm-2 (22 %).