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In cellular wireless location, time of arrival (TOA) is sometimes corrupted by non-line-of-sight (NLOS) error due to multi-path propagation, which results in poor location accuracy. So, NLOS error is the main difficulty for mobile user location in wireless communication system. In this paper, we suggest a new NLOS error mitigation algorithm based on Kalman filter. Both the true value of TOA and NLOS error are served as the state variances of Kalman filter, according to the construction features of the former and the statistic characteristics of the latter, we theoretically deduce the condition on which we can obtain the unbiased estimation of the true value of TOA. At the same time, the paper fixes on the state transition matrix of Kalman filter. At last, we take the situation when NLOS is exponential for example, the simulation results show that the approach we present need not detect NLOS error, and the location performance is improved with better estimation accuracy and higher calculating speed.