The angular distribution of isotopic sputtering was studied theoretically and experimentally in this paper. A careful experiment was carried out, and a theoretical model was established. The result was that the light isotopes are preferential ejected "normally" and the heavy ones "obliquely". The reason was considered to oe the energy and momentum asymmetry between light and heavy isotopes in the collision cascades. A simple formula of the angular distribution of isotopic sputtering which fits the experimental result is given in this paper.