The purpose of this study is to improve real-time compensation of machining errors caused by deflection of two-flute end mills with small diameter like ball end mills. For this purpose, relation between tool deflections at cutting point and forces applied to cutting point was investigated, and then the measured machining errors was compared with the machining errors estimated from measured cutting forces. As a result, the estimated machining errors approximately coincided with the measured machining errors but the difference between the estimated and measured machining errors was larger at smaller axial depth of cut in down cut and it was larger at larger axial depth of cut in up cut.