The grain structures and microscopic features of a nickle-base Inconel 738LC were examined under various melting and casting conditions. Results show that the gain size of the g matrix is reduced significantly by a treatment of lowing melt superheat temperature (LMST). By the addition of a trace intermetallic compound A1xNiy to the melt together with LMST treatment, a remarkable refinement could be obtained. The grain size over the whole section of castings could be refined to 0.5-0.8mm, i.e. to ASTM grain size M11-12. In addition, the size of primary MC carbides in the alloy is reduced. The secondary dendrite arm spacing and phase constitution of the alloy are less sensitive to the additions.