The effects of initial powder size on microwave-assisted sintering (MWS) were investigated. BaTiO3 powders with an average particle size of 50, 100, and 500nm were prepared and sintered with MWS and conventional heating-based sintering (CS). Samples of the 50 - and 100-nm-sized BaTiO3 powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50-nm-sized BaTiO3 powder resulted in a relative mass density of more than 90% when sintered at 1050 degrees C, whereas the same density was achieved at 1200 degrees C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500-nm-sized BaTiO3 powder was used. The sinterability of the BaTiO3 ceramics prepared through the MWS of mechanically milled, 50-nm-sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO3 ceramics prepared through the MWS of the 100- and 500-nm-sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of similar to 50-nm-sized powders, but is negligible for 500-nm-sized powders.