Single-phase β-Zn4Sb3 bulk sample and several samples based on β-Zn4Sb3 with different amounts of Zn were produced by melting in an evacuated quartz ampoule followed by slow cooling and their thermoelectric properties were investigated at 300 to 700K. The distribution of Zn in the samples was observed and the effects of excess of Zn on thermoelectric properties of β-Zn4Sb3 compound were investigated systematically. With the amount of Zn increasing, electrical and thermal conductivities of the sample increase, and the Seebeck coefficient declines. The excess of Zn leads to an obvious improvement to the power factor of the β-Zn4Sb3 based materials. The ZTmax of the sample is 1.10 at 700K when the amount of excessive Zn is 2at%.