Objective An effective but simple method for quantifying the fracture healing progress was provided to obtain more information on fracture healing mechanics from plain X-ray film. Method Twelve rabbits were used for experimentally osteotomy at the middle tibiae, each of which was fixed by the sliding fixator with four pins of 1.5 mm diameter, with mini transducer of displacement measuring the sliding micro movement between the fractured bone fragments, denoted as ΔL. The bending rigidity, denoted as K=P/ ΔL, of the rabbits’ tibia (with sliding fixator together) were recorded by computer data assembling system once a week, where P was the load on the fracture site from anterior of the tibia by manual operation of a loading transducer. The anterior posterior and lateral radiographs were also taken every week by digitally upgraded X-ray radioscope. The effective weakest transverse curved interfaces were found, and the equivalent bending moments of inertia were calculated from the radiographs automatically, by gray scan along the longitudinal axis of the tibia upon the image programs edited in Delphi programming environment. And they were used for quantitative description of the rigidity of the healing bone. Results The sliding fixators were removed in 29~41 days after the osteotomy. Ratios of the minimum to the maximum of equivalent bending moments of inertia for each fracture bone were calculated. And they were found to increase via the healing time. The means of the ratios of all twelve rabbits calculated from lateral radiographs were (0.31±0.17) for the first week, (0.34±0.13) for the second week，(0.43±0.20) for the third week, and (0.56±0.23) for the fourth week after the osteotomy, respectively. From anterior posterior radiographs, the means of the ratios were (0.40±0.19), (0.47±0.16)， (0.56±0.20), and (0.66±0.11), respectively. These ratios were significantly correlated with the bending rigidity K of the fracture tibia of the rabbits. The means of K value was （3.976±4.986）N?μm-1 before the operation，and （0.679±1.026）N?μm-1 for the first week，（2.115±3.233）N?μm-1 for the second week，（3.459±4.723）N?μm-1 for the third week and （4.788±4.831）N?μm-1 for the fourth week after the operation, respectively. Conclusions The growth of external callus has its own rules. It can be distinguished from plain radiographs easily, and it grows from either side of the fracture site and meets together gradually, filling up the gap at the fracture site at last. The methods described in this paper could reflect the feeling when reading the radiographs.