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.