The peculiar nonlinear error in the 5axis NC machining is studied. An improved tool step determination method for the 5axis NC machining is proposed, in which the nonlinear error is compensated. Both the arcchord error and the nonlinear error are considered when computing the tool step, and the comprehensive error is taken as the function of the tool path parameter. Due to the difficulty of gaining the derivative of this function and the tool step according to the prescribed tolerance, the secant method is improved to guarantee the real solution locating in the converging range which is adopted to compute the tool step. The implementing example shows that the proposed method can compensate the nonlinear error effectively with meeting the prescribed tolerance. The exact tool step can be gotten efficiently with the improved secant method.