Effects of a liquid-phase-sintering aid, BaCuO2 + CuO (BCC), on densification and microwave dielectric properties of (Zr0.8S0.2)TiO4 (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST, The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO2, SnO2, CuO, and BaTi8O16 which reduce the amount of BCC flux available for liquid-phase sintering, The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST, For samples with 2.5-5 wt% BCC, a high relative sintered density is obtained at 1000 degrees C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35-38 and 2800-5000, respectively.