As a finer grain structure could impart better weldability and mechanical properties, the effects of various casting parameters on the grain structure of primary gamma were examined for the investment casting of Inconel718. The grain size of cast specimen became smaller as the superheat of pouring melt decreased from 110K to 20K and the superheat below 20K resulted in a very fine grain structure. The mold dressed with cobalt-aluminate (CoO-Al2O3) gave a slightly finer grain structure than that with a conventional facing material. A thin section casting poured at a lower temperature showed a fine gram structure and high soundness, and the computer simulation confirmed the low temperature pouring process could be successfully applied to the investment casting.