Disclose are a reaction container and a vacuum heat treatment apparatus. A method of preparing a reaction container comprises preparing a graphite mixture by mixing first and second graphite powders having particle sizes different from each other, preparing a graphite molded body by pressing the graphite mixture, and processing the graphite molded body. The density of the graphite molded body is in a range of 1.8 g/cm3 to 2.1 g/cm3. A method of preparing a reaction container comprises preparing a graphite molded body by pressing graphite powders, and processing the graphite molded body to prepare the reaction container. A carbon source is impregnated into the graphite molded body or the reaction container, and density of the reaction container is in a range of 1.8 g/cm3 to 2.1 g/cm3.