Isothermal transformation behavior of a 10% Cr heat resistant steel fabricated by centrifugal casting process was investigated. Normalized specimen at $1100^{circ}C$ for 1 hour was isothermally annealed at temperature range between $600^{circ}C$ and $700^{circ}C$ with various time. The annealed specimen had eutectoid structure which was generated along austenitic grain boundary during isothermal annealing. Areal fraction of eutectoid structure increased up to 25% after holding at $700^{circ}C$ for 20 hours. It was observed that austenitic matrix was transformed to ferrite structure and fine $M_{23}C_6$ carbides with increase of annealing time. Time-temperature-transformation diagram of the centrifugally cast 10% Cr steel with 0.18 wt% C was plotted based on the results of isothermal transformation behavior.