The whole-body scanning of fluorine-18-fluoro-2-deoxy-D-glucose (18F-FDG) combined with positron emission tomography/ computed tomography (PET/CT) is considered an important modality used for diagnosis, tracking, and staging of the oncological as well as neurological diseases. On the other hand, the interest is rising as a concern of the radiation exposure for patients during PET/CT because of the higher radiation dose as compared to other imaging modalities. This study aims at estimating the patient-specific effective dose for F18- FDG PET/CT studies using the patient's size-dependent correction method at a single center in Riyadh, KSA. Patients and methods: A total number of 80 patients18F-FDG examinations were recorded. For each patient, the following parameters were recorded the age, sex, weight ,height, body mass index (BMI), CT volume dose index (CTDIvol), the dose length product (DLP) , and finally the net injected FDG activity. The total patient effective dose was then recorded and evaluated. Results: The results show that the mean effective dose for the PET scan for the patients scaled to their body weight was 8.36±1.2 mSv. On the other hand, for the CT part, the patient effective dose estimated by using the DLP method was 26.8% higher than that estimated by the SSDE method which directly involve the patient size in its calculations. The total patient's effective dose resulting from using the DLP method was 18.886 mSv and by using the SSDE method it was 15.138 mSv. Conclusion: The patient's effective dose estimate from the PET scan should be calculated by considering the weight-scaled. The estimated patient effective dose from the CT scan during PET-CT F18-FDG using the DLP method was 26.8% higher than that estimated by the SSDE method. The patient's effective dose from the CT part was higher than that of the PET part.