Magnetization measurements of CsCuC13 is performed using pulsed high magnetic fields at various temper- atures below TN. Magnetization shows a small jump at 12.5 T and saturation at 31 T at 1.1 K when Ho // c, while a small plateu at around 12 T when Holc. The results are quite different from those reported so far in other triangular lattice substances and seem to be due to other type of incommensurate phase transition. Hexagonal ABX3 type compounds show compli- cated but interesting magnetic behaviors due to trian- gular lattice structure, where A = Rb, Cs, X = C1, Br and B = Mn, Fe, Co, Ni, Cu. They have been inves- tigated for more than two (l) but still remain some attractive problems. ype of crystal struc- ture, magnetic moments couple/each other strongly along the c-chain and then these compounds can be regarded as quasi-one dimensional magnets. The inter- chain exchange interaction is usually -weak compared to intrachain one. Because of the triangular network in the c-plane, the moments whose in-plane interaction is antiferromagnetic have frustration and then spin struc- ture becomes complicated. CsCuC13 is one of the materials of this category 121. Strong intrachain ferromagnetic exchange inter- action J1 .= 16.7 cm-' makes almost ferromagnetic but low pitch helical spin structure along the c-axis below TN = 10.6 K. This is considered to be due to the antisymmetric Dzyaloshinsky-Moriya interaction. In the c-plane, the-spin system shows a 120' structure caused by weak antiferromagnetic interchain interac- tion J2 = 2.7 cm-l. The magnetic moment lies in the plane due to D-M interaction whose vector is parallel to the c-axis, although dipolar anisotropy apparently makes spins prefer to direct to the c-axis.