The effect of thermal drying on the efficiency of clay removal from fine colemanite ores was investigated. Clay minerals were removed by wet screening with or without mechanical action such as tumbling and scrubbing. Tests showed that drying colemanite ore samples above 100 degreesC prior to screening and mechanical action substantially increased B2O3 content in the washed product. According to the tests conducted with -25+3.36 mm size fraction, B2O3 content of the clean product rose up to 42.3% compared to 39.6% that obtained with no heat treatment. The B2O3 grade of the screened product for -3.36 size fraction, however, increased up to 38% after drying at 200 degreesC whereas drying at room temperature only yielded 33.1%.