This paper is devoted to the effect of sodium sulfate as negative paste additive on the performance of the lead-acid battery. Six different percentages of sodium sulfate were added to negative paste. The effect of sodium sulfate on discharge capacity, cycle life and cold cranking ability of the sealed lead-acid batteries were investigated. Batteries containing sodium sulfate in negative plates at low amount (0.1wt%) showed a remarkable electrical behavior during the test. Results indicate that negative electrodes containing 0.1wt% sodium sulfate exhibit discharge capacity of the more than 3% and 12% with respect to negative electrodes without sodium sulfate before and after cycling test, respectively. Addition of sodium sulfate also increases the time of reaching to cut off voltage of 6V at cold cranking test more than 17%. The main effect of sodium sulfate is to increase the cycle life of the lead-acid batteries to more than 18%. Scanning electron microscopy (SEM) was used for the investigation of paste morphology.