A digitization approach to the time course of radiation-induced mouse leg contracture was proposed for quantifying the radiation effect on an individual living mouse. The shortening of the mouse leg length can be easily measured with a caliper/ruler to offer a very simple digitalized index of the radiation effect. Left hind legs of mice were irradiated with single dose of 32 Gy of 290 MeV carbon-ion beam using 0, 50, or 117 mm binary filter (BF). The right legs were used as a control. The lengths of both hind legs of the mice were measured using a digital caliper before irradiation and every week after irradiation. The degree of leg contracture, ΔSt, at the time point t was estimated by subtraction of the left irradiated leg length from the right control leg length. Equation was fitted on the daily time course of ΔSt, and two parameters, ΔSmax and Ts, were estimated. ΔSt=ΔSmax×(1-exp(t/Ts)), where ΔSmax is the maximum degree of leg contracture, and Ts is time of leg contracture. The effect of carbon-ion irradiation on a living mouse was quantified by ΔSmax and Ts of the leg contracture, and then compared to that of X-rays. By 32 Gy irradiation, ΔSmax was largest for the BF117 experiment, followed by X-ray~BF50>BF0. Ts was shortest for the BF50 experiment, while other irradiation conditions give similar Ts. A logarithmic function was successfully repurposed for the evaluation of radio-biological response.