The present invention relates to a heat-resistant resin and a method of preparing the same. More particularly, the present invention relates to a heat-resistant resin including 100 parts by weight of a styrene based resin and 0.5 to 5 parts by weight of silica having a contact angle of 10 to 60° and an average particle diameter of 0.1 nm or more and 100 nm or less, a method of preparing the same and a heat-resistant resin composition including the same. In accordance with the present invention, provided are a heat-resistant resin having a low moisture content and a small content of fine particles while having heat resistance equal to or higher than conventional resins and, accordingly superior cohesion, a method of preparing the same, and a heat-resistant resin composition including the same.