We have fabricated p- and n-type resistor stress sensors on (111) silicon surface and investigated the strain effects, which were generally ignored in previous works, for the precise stress measurements. We obtained the corrected values of the pi-coefficients for p- and n-type silicon by considering the strain effects, without which more than 25% discrepancies may be induced for small pi-coefficients. We observed that ignoring the strain effects was one of the potential errors which induce large discrepancies in stress measurements, especially for n-type stress sensors on (111) silicon.