To identify effects of the bottom inflow area on pool boiling heat transfer in a vertical annulus, a heated tube of 25.4mm diameter and the saturated water at atmospheric pressure has been tested. The inflow area has been changed from 0 to 1061.4 mm². To clarify effects of the inflow area on heat transfer results of the annulus are compared to the data of a single unrestricted tube. According to the result, the existence of the inflow area changes heat transfer coefficients much comparing to the single tube. It shows most improved heat transfer as the area ratio Ar is 0.372. The cause of the tendency is explained by the combined effects of the liquid agitation increase at low heat fluxes and bubble slug formation decrease at high heat fluxes.