We succeeded in growing high-quality single crystals of UCoGa5 and URhGa5 by the self-flux method and measured the electrical resistivity, magnetic susceptibility, and de Hass-van Alphen(dHvA) oscillation. Three kinds of dHvA branches were observed, ranging from 8.6x 105 to 8.1 x 106Oe, which correspond to closed Fermi surfaces. UCoGa5 and URhGas are concluded to be semimetals. The cyclotron mass is small, ranging from 0.6 m 0 to 2.9 m 0. © 2002 Atomic Energy Society of Japan.