Synthesis method of Zr2Al3C5 via solid state reaction between Al, ZrC and carbon powder was studied. Al-ZrC-C compact with equivalent mol ratio of Zr2Al3C5 was heated up to 1600oC in Ar atmosphere for 1 h and 4 h but ZrC phase still existed as major phase with very small amount of Zr2Al3C5. Because ZrC started to oxidize at low temperature under very low oxygen partial pressure, the same mol ratio of Al-ZrC-C compact was heated at 1600oC in vacuum for 1 h as parallel test. After firing in vacuum, some carbon still exist with small amount of AlZrC2 occurred with Zr2Al3C5 as a main phase, but no ZrC was found. Different result from firing in Ar atmosphere and in vacuum had been discussed here. It was believed that very small amount of impurities in Ar had some effect on the formation of Al-Zr-Ccompound. The effect of very small amount of impurities in Ar was studied by thermal analysis (DTA/TG) and XRD. It was found that very small amount of impurities in Ar haseffect on the reaction between Al, ZrC and carbon by diffusion through the surface and formZr-C-O-N solid solution. This solid solution cannot differentiate from ZrC by XRD. With help ofthermal analysis method (DTA/TG), Zr-C-O-N solid solution can be differentiated from ZrC.Therefore, synthesis of Al-Zr-C compound should be done in vacuum. Zr2Al3C5 can be prepared from mixture of Al-ZrC-C with excess amount of Al at 1600oC for 1 h.