Abstract: Acccording to the principle of thick target, we used the aqueous solutions of uranyl chloride of various concentrations as thick targets and platinum plates of known surface area as absorbers immersed in the target solutions. The ranges of the U(n,f) fission fragments 89Sr, 91Y, lioBa, 141Ce and 144Ce in the'aqueous solutions of uranyl chloride of various concentrations were determined. In the concentration region of 0.16 U%-6.2 U%, the uranium concentration had no significant effect on the measurement of the range. Therefore, the ranges of the fission fragments in dilute U02C12 solutions are very close to that in pure water, and the mean value of the ranges in U02C12 solutions of various concentrations was taken as the range in water. The experimental result of the ranges of these five fission fragments in water were: RSt-90 = 2.39±0.04mgcm-2, Ry-91 = 2.35±0.09mgcm-2, RBa-140 = 1.92±0.07mgcm-2, RCo-141=1.91±0.12mgcm-2, Rc0-144=1.84±0.10mgcm-2. In order to estimate the effect of back scattering of fission fragments in platinum plate, we did the experiments using stainless steel plate as absorber (the aqueous solutions of uranyl chloride as thick targets). The results were similar. Thus, the effect of back scattering was not significant. This work provides a convenient means for determining the ranges of the fission fragments in a liquid.