The purposes of this study were to develop the computer program for the contrast and geometric correction in digital subtration radiography with the IDL(Interactive Data Language) and compare the results with this program for the correction of the non-standardized radiographs to those of standardized radiographs and those with "Emago" software, the commercial program for the correction. The procedures were written for the contrast correction and subtraction with the geometric correction, using IDL. 32 pairs of periapical radiographs of premolar and molar portion of two dry human mandibles were taken at two different occasions with XCP film holder(non-standardized films) and another 32 pairs with customized XCP film holder(standardized films). Subtraction of standardized film pairs was performed. Subtraction after the contrast and geometric correction of non-standardized films was performed using the newly developed program and Emago software. Standard deviations of grey levels of the subtracted images by the newly developed program were compared with those of the standardized group and Emago-corrected group. Standard deviations of grey levels of new programcorrected group were much smaller than those of the Emago-corrected group(p<0.001) and slightly larger than those of standardized group(p<0.05). However, the difference was very minute. This study indicates that the newly developed program written with IDL may substitute the mechanical standardization for digital subtraction radiography.