Study on thermal decomposition of yttrium oxalate produced by reaction crystallization of yttrium chloride and oxalic acid was carried out. The yttrium oxalate having three crystal waters belonged to monoclinic crystal system and the crystal waters of yttrium oxalate were dehydrated before the yttrium oxalate was thermally decomposed. The dehydration of crystal water in yttrium oxalate did not changed the molecular bonding and crystal structure of yttrium oxalate but gave significant influence on shape and size of yttrium oxalate crystal. Thermal decomposition of yttrium oxalate occured above 366℃. Between 366-465℃ the yttrium oxalate was thermally decomposed and it resulted in a non-stable and non-crystalline product. Above 500℃, however, yttrium oxide having cubic crystal system was produced by the thermal deccomposition. By using Hyper-Chem it was predicted that the yttrium oxalate synthesized by the reaction crystallization and the yttrium oxide produced by the thermal decomposition had three dimensional molecular structures, which were preferred thermodynamically. The results of thermal decomposition of yttrium oxalate under nitrogen gas were same with those under air.