The improvement of the critical current density J(c) of YBCO thick films has been attempted by adding BaSnO3 powder and ultrafine Sn particles, whose diameter is about 2 mu m and 7 X 10(-2) mu m, respectively. It was found that the addition of a small amount of these particles was effective for the enhancement of J(c) of thick films prepared by a liquid-phase processing method. The 1 wt.% BaSnO3 films fired at T-s=1040-1060 degrees C and the 3 wt.% Sn films (T-s=1030-1060 degrees C) showed J(c) Values (77 K, 0 T) of about 2.1-2.4 X 10(3) Acm(-2) and 3.1-3.5 X 10(3) Acm(-2), respectively, as compared to 2.0 X 10(3) Acm(-2) for the undoped films.