The present study aims to identify the effect of sintering atmosphere [O2, 75N2-25 H2 (mol%) and H2] on microstructural evolution at the relatively low sintering temperature of 1040oC. Samples sintered in O2 showed a bimodal microstructure consisting of fine matrix grains and large abnormal grains. Sintering in 75 N2 - 25 H2 (mol %) and H2 caused the extent of abnormal grain growth to increase. These changes in grain growth behaviour are explained by the effect of the change in step free energy with sintering atmosphere on the critical driving force necessary for rapid grain growth. The results show the possibility of fabricating (K0.5Na0.5)NbO3 at low temperature with various microstructures via proper control of sintering atmosphere.