We carried out the de Haas–van Alphen (dHvA) experiment for an antiferromagnet UCd11 and its non-5f reference compound ThCd11 to clarify the Fermi surface properties and the true electronic specific-heat coefficient γ of UCd11. We observed dHvA signals of both compounds and compared the experimental results with the result of theoretical band calculations. The Fermi surfaces of both compounds are found to be small and closed. The cyclotron masses obtained from the dHvA data of UCd11 are compared with those of reference compounds ThCd11, NpCd11, and LaCd11, and the γ value of UCd11 is estimated to be about 100 mJ K−2 mol−1, which is much smaller than the 840 mJ K−2 mol−1 in the previous reports. It is concluded that this large value is mainly not due to heavy quasiparticles but due to the magnetic contribution to the specific heat.