[en] Polycrystalline YbB_x (x = 5.7-6.3) were prepared and their thermoelectric properties were measured to discuss the possibility of p-type thermoelectric material. P-type Seebeck coefficient was observed at around room temperature in the specimens with nominal composition of x=6.1 and 6.3, though they turned into n-type at high temperature due to thermal excitation of n-type carrier. Seebeck coefficient and electrical conductivity of the p-type YbB₆ were as high as those of n-type alkaline-earth hexaborides, suggesting the possibility of p-type counterpart to the n-type ones.