We succeeded in growing a single crystal of USi3 with the cubic crystal structure and measured the transverse magnetoresistance and de Haas-van Alphen(dHvA) oscillation. Fifteen dHvA branches were observed, ranging from 7.37x10(6) to 2.33x10(8) Oe. These branches are in good agreement with theoretical ones based on 5f-itinerant FLAPW band calculations. Cyclotron masses, which are in the range from 0.39 to 4.17 m(0), are approximately the same as the corresponding band masses. These results indicate that the itinerant 5f-electrons form wide bands. The magnetoresistance results confirmed the existence of open orbits, which can he well explained by the band 10-electron Fermi surface.