Background: Concern about radiation risk of computed tomography (CT) scan as a diagnostic modality has increased in recent years. Diagnostic reference levels (DRLs) is one of the tools to optimizing radiation dose of patients. CTDIv (Volume Computed tomography dose index) and DLP (Dose Length product) are used for assessment of DRLs. The CTDIv under/overestimate the patient dose. AAPM has introduced SSDE (Size-specific dose estimates) for estimation of patient. In this study, the DRLs of head and abdomen-pelvis CT examinations of adults is determined using CTDIv, DLP and SSDE. Materials and Methods: 680 CT examinations of head and abdomen-pelvis were collected from PACS (Picture archiving and communication system) in Imam Khomeini and Mostafa Khomeini hospitals. The D_eff, CF and SSDE calculated using AAPM TG-204 and TG-220. Statistics analysis calculated using SPSS version 18. Results: For abdomen-pelvis third quartile of CTDI_V, SSDE and DLP was 9.96, 13.58 and 527 and values of 27.62, 26.79 and 402.90 are determined for head, respectively that are lower than national DRLs. Also, calculated conversion factor (CF) for head and abdomen-pelvis was 0.97 ± 0.75and 1.45 ± 0.17, respectively. Conclusion: DRLs were lower than other studies in this study. Using the AEC (Auto Exposure Control) and different kVp in this hospitals can help optimization of patient dose. The SSDE must be calculable by radiographers to more accurate estimation of patient dose using CFs.