Yttrium aluminum garnet (YAG) powders were synthesized via mechanochemical solid reaction using $Y_2O_3$ with three types of aluminum compounds. $Y_2O_3$ reacted mechanochemically with all A1 compounds and formed YAM (yttrium aluminum monoclinic), YAG and YAP (yttrium aluminum perovskite) phases depending on the starting materials. The ground samples containing ${gamma}-A1_2O_3$ showed the best reactivity, whereas the ground sample containing A100H, which had the largest surface area, exhibited pure YAG after calcination at $1200^{circ}C$. The sample containing Al had the least reactivity, producing YAP along with YAG at $1200^{circ}C$. The types and grinding characteristics of the starting materials and grinding time are believed to be important factors in the mechanochemical synthesis of YAG.