SummaryBackground/Aim: The determination of remaining dentin thickness (RDT) is quite critical in providing appropriate treatment indications especially in deciduous molars. The aim of this study was to investigate the accuracy of in vitro pre-operative radiographs before caries removal in RDT measurement.Material and Methods: This in vitro study was conducted with 30 lower deciduous second molars with deep approximal dentinal caries. Following the standardized digital radiographs were obtained, RDT was measured by determining the closest distance between the carious lesion and the pulp chamber on the computer software. After caries removal, the samples were sectioned in mesio-distal direction and photographed. RDT was measured on photographs similarly to radiographic measurements. In statistical analysis, Interclass Correlation Coefficient (ICC), Wilcoxon-Signed and Bland Altman Plot (BAP) tests were performed to determine the agreement and statistically difference.Results: The mean RDT value in photographs was found to be 10% higher than measured on radiographs. Although, correlation coefficient was within confidence interval, both RDT values showed good but not excellent agreement (correlation coefficient= 0.812). Also, BAP test did not demonstrate strong agreement. In descriptive statistics, significant difference was found between two measurements (p<0.05).Conclusions: Pre-operative radiological examination can not alone provide reliable results regarding the accurate determination depth of caries lesions. Clinicians should also consider operative diagnostic criteria for appropriate treatment options in deciduous teeth.