Abstract:The mathematical model of bending logs of curved elliptical cylinder is analysed with the elliptical-archimedes spiral envelope in cylindric coordinate. This kind of logs are definited ar C-typed bending. Based on the measurement of bending by log length and the bending bow back, this papre proposes a formula, by which the bending log volume with practically measured parameters can be calculated. Therefore, the loss of log voeume resulted from bending in calculation bas been quantitatively reduced. with this method and theory the optimum wood recovery of bending logs and optimum cutting of bending tree length can be obtained. This provides a thcoretical base for working out software of numerical control in sawing.