In order to reduce the toxic emission in the exhaust gas and to improve the fuel efficiency of automobile gasoline engines, the maximum gas temperature has been raised from 1173K to 1273K, further to 1323K in the future. As the castings for exhaust gas components, ferritic heat resistant or ferritic stainless steels have the adequate performance under 1173K, but they lose the strength over 1173K. Austenitic cast steels are serviceable up to 1253K, but none was able to endure the severe thermal fatigue over 1273K.The thermal fatigue resistance generally gets better with higher strength and lower thermal expansion. In this study, authors extracted the effects of alloying elements on both properties for the experimental austenitic 12 cast heats. They found that W, C and Nb increase high temperature strength, and W, Ni, C, Cr and Nb decrease mean coefficients of thermal expansion.Based on the above they designed new austenitic cast alloys, 0.25/0.30C, 0.8/1.0Si, 20/25Cr, 13/40Ni, 0/6W, 0.7/1.5Nb, Bal. Fe. Their thermal fatigue resistance was proven by the fluidized bed test truly excellent and unrivaled as predicted by FEM analysis.