The magnetoresistance and the de Haas-van Alphen (dHvA) effect in CeSn3 have been measured in the field up to 150 kOe and at temperature down to 0.45 K. The magnetoresistance does not show a tendency to saturate in all field directions. This result claims that CeSn3 is a compensated metal with an equal carrier concentration of electrons and holes, and electron and hole Fermi surfaces possess no open orbits. In contrast to an uncompensated metal LaSn3, the 4f electrons in CeSn3 become itinerant electrons. This is consistent with the dHvA data which are almost explained by the modified Fermi surface model based on the result of band calculation where the 4f electrons are treated the same as the usual s, p, d conduction electrons. Fermi surface, CeSn3, magnetoresistance, de Haas-van Alphen effect. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.