In IF steel without interstitials, the Hall-Petch coefficient (ky) is low as about 100 MPa･μm1/2 but it is enhnced by adding some alloying elements. Author et al. has already reported that a small amount of carbon increases ky greatly while nitrogen does not give influence so much. On the other hand, it is already known that phosphorus has a tendency to segregate at grain boundary of polycrystalline ferritic iron and that phosphorus also gives some influence to ky. However, the effect of carbon is not considered on the evaluation of ky in steels containing phosphorus. In this study, effect of phosphorus on the ky of ferritic iron was investigated by varying the grain size in Ti bearing IF-steel with different amount of phosphorus. Besides, the concentration of phosphorus and carbon at grain boundary was estimated by the grain boundary segregation model proposed by Seah et al. and the interaction between phosphorus and carbon was discussed in connection with the change of ky.