A castable invar alloy based on the Fe - Ni - Co - C system is developed. The highest average values of the coefficient of linear thermal expansion of the alloy are 5.2 x 10(-6) K(-1), 6.3 x 10(-6) K(-1), and 7.4 x 10(-6) K(-1) in the temperature ranges of 20 - 400, 20 - 450, and 20 - 500 degrees C, respectively. The alloy is designed for mold casting of parts with elevated heat resistance including those for operation in a couple with ceramics, carbon plastics, and other polymer composite materials (PCM).