Abstract. Let U (2) and U (2, 1) be the pair of unitary groups over a global field F and π an irreducible cuspidal representation of U (2) which satisfies a certain L-function condition. By using a regularized Siegel-Weil formula, we can show that the global theta lifting of π in U (2, 1) is non-trivial if every local factor πυ of π has a local theta lifting (Howe lifting) in U (2, 1)(Fυ ).