Introduction: Periotest is a device that measures the damping characteristics of the periodontium. It has a tapping head which percusses the tooth 16 times in 4 seconds. This sensitive tip records the duration of contact with the tooth and gives us a reading. The Periotest value (PTV) is ranging from -8 to +50. The more mobile the tooth is, the contact time will be longer and the Periotest value will be higher. Periotest is more commonly used in implant dentistry to evaluate implant stability, however there are studies that used the Periotest device to evaluate the outcome of endodontic surgeries, evaluating anterior teeth after trauma and in follow-up visits, and diagnosis of replacement resorption. Percussion test is a part of routine endodontic testing, and the results are subjective and may vary between operators. In addition, pulp necrosis may lead to local bone destruction, less periodontal support of the affected tooth and potentially more mobility. It has been shown that Periotest evaluation provides an objective indication of the extent of periodontal bone loss. Therefore, application of an objective percussion test using this device may provide more accurate diagnostic data regarding the supporting tissue. Purpose: The purpose of this study was to compare the Periotest value in teeth with necrotic pulp and periapical involvement to their healthy contralateral tooth. Materials and Methods: After doing routine endodontic testing, 49 pairs of teeth were included in the study. The inclusion criteria were teeth with necrotic pulp tissue with periapical lesion that needed root canal treatment and have a contralateral tooth with normal pulp and normal apical tissue. Teeth with severe periodontal disease, crown or bridge restoration, teeth in crowded areas, teeth with history of recent trauma or extensive caries were excluded. Periotest device was used to do horizontal percussion test on the tooth with pulp necrosis and its healthy contralateral. The data recorded for each sample. Data was analysis using non-parametric Man—Whitney test to compare test and control groups, with p<0.05 as significant. Results: The average Periotest value (PTV) in the necrotic teeth group was 6.94 and the average in the healthy contralateral teeth was 2.34. Statistical analysis shows a significant difference between the two groups (P value <0.0001).Statistical Analysis of the data by tooth type shows significant difference when comparing the PTV of necrotic molars, premolars, and anterior teeth to their healthy contralateral teeth. Tooth type (post/Ant), and pulp necrosis are significant predictors of PTV, however, Prescence or absence of caries is not a predictor of PTV. PTV of the necrotic teeth with periapical lesion was higher than their contralateral tooth in 93.8% of the cases, equal in 2% of the cases and lower in 4% of them. Conclusion: Periotest may be useful as an objective tool for percussion test in endodontic diagnosis and it may provide needed information regarding the health of the periapical tissue. This device is potentially useful in evaluating the bone healing in periapical region following root canal treatment.