Statement of Problem: Alveolar bone level measurement in periodontal disease is very important because it may affect planning of forthcoming surgery, treatment response and prognosis. Surgical approach is the most accurate procedure for obtaining this measurement. Nowadays, transgingival probing as a non-invasive and accurate method, has been claimed to be a proper substitution for surgical measurement and direct visualization.Purpose: This study aimed at evaluation of diagnostic value of bone sounding technique in estimation of bone level in I, II and III- wall periodontal defects.Materials and Method: Ninty vertical defects in 37 individuals with chronic periodontitis having either one, two or three-wall defects were assessed. Bone levels were measured by UNC-15 probe. The resulted values were compared with the values obtained during surgery. Collected data were analyzed using Paired samples T test and Pearson’s correlation coefficient.Results: High and significant correlation was found between the results of transgingival probing and surgery methods (0.97, 0.94 and 0.95 in one, two, and three-wall defects respectively (p<0.001)). The mean bone levels measured by transgingival probing were 0.21, 0.53 and 0.19 mm less than their assigned values in surgery and there was no significant difference between one and three- wall defects (p<0.001).Conclusion: The findings of this study indicated a high correlation between the two procedures, bone sounding and surgery, in detection of bone level. Due to the negligible difference, it seems that transgingival probing could be considered as a feasible replacement for invasive surgical approach in detection of alveolar bone level.Key words: Bone sounding, Periodontal defects, Bone Level.