Cs2CuBr4 is an S = 1/2 quasi-two-dimensional frustrated antiferromagnet with a distorted triangular lattice parallel to the bc-plane. Cs2CuBr4 undergoes magnetic ordering at TN = 1.4 K at zero magnetic field. In the ordered phase below TN, spins lie in a plane that is almost parallel to the bc-plane and form a helical incommensurate structure with ordering vector Õ 0 = (0, 0.575, 0). The incommensurate spin structure arises from the spin frustration on the distorted triangular lattice. The magnetization curve has a plateau at approximately one-third of the saturation magnetization for magnetic field H parallel to the b-and c-axes, while no plateau is observed for H a. The ordering vector Õ 0 increases with increasing magnetic field parallel to the c-axis, and is locked at Õ 0 (0, 2/3, 0) in the plateau region, which indicates that the up-up-down spin structure is realized in the plateau state. The magnetization plateau should be attributed to quantum fluctuation. For H b and H c, the second anomaly suggestive of tiny plateau is observed at roughly two-third of the saturation magnetization. The magnetic field versus temperature diagram is presented. Small amount of Cl − substitution for Br − produces drastic suppression of TN. With increasing Cl − concentration x, the magnetic ordering disappears at x 0.17. It is also observed that in Cs2Cu(Br1−xClx)4 phase transition smears with increasing external field and disappears, irrespective of field direction. This should be attributed to the random field effect.KEYWORDS: Cs2CuBr4, Cs2CuCl4, Cs2Cu(Br1−xClx)4, spin frustration, triangular antiferromagnet, quantum fluctuation, helical magnetic ordering, magnetization plateaus, disorder, random field effect IntroductionTriangular antiferromagnets (TAF) have been of great interest from the viewpoint of the interplay of spin frustration and quantum fluctuation. In most of conventional antiferromagnets, which are described by two-sublattice model, the ground state is determined by the classical energy, and the quantum fluctuation gives only small correction to the ground state energy. On the other hand, in Heisenberg TAF, spins form the 120• structure in the ground state due to the spin frustration. However, in a magnetic field, spin structure of the ground state cannot be uniquely determined by the classical energy only, and thus, the ground state has continuous degeneracy in the magnetic field. No phase transition occurs up to saturation, so that the magnetization curve is monotonic. For quantum Heisenberg TAF with small spin S, the quantum fluctuation plays an important role in determining the ground state, because the quantum fluctuation can remove the continuous degeneracy of the classical ground state. The quantum fluctuation in TAF was discussed using the spin wave theory, which describes the spin system