In order to obtain behavior and failure mechanisms of high strength steel endplate connections in fire, full-scale tests on 7 joints were conducted at elevated temperature 550℃ under steady state fire condition. The connection specimens included 4 high strength steel endplate connections and 3 mild steel endplate connections. The endplate thickness and the endplate material are the focus of the parametric study. In order to compare, corresponding tests at ambient temperature were carried out as well. Moreover, the current provisions of Eurocode 3 were validated with experimental results. It is found that a proper thinner high strength steel endplate can improve the rotation capacity of connection both at ambient temperature and in fire, and simultaneously achieve similar moment resistance with a mild steel endplate connection, which guarantees the safety of an entire steel structure.