Abstract: Aim To analyze the finite register length effects based on TMS320C62x whose multiplier is 16 bit. Methods The word length error using truncation of sign magnitude code was analyzed on the unit circle of complex plane. The computer simulations using truncation and rounding of sing magnitude code and complementary code were given. The effect of noise on finite register length error was discussed.Results When using linear phase signal and algorithm of truncation of sign magnitude code, the absolute values of output in frequency domain decrease because of the word length error of r steps' FFT. The error's mean and variance are both linearly increased with r . The mean of errors can be decreased by adding the white noise to the signal. Conclusion The result of simulations based on TMS320C62x's simulator are in agreement with the theoretical analysis.