Background: Use of a fixed threshold value for tumor volume delineation in positron emission tomography (PET) images will ignore the effect of size of the lesion and source to background ratio (SBR). The purpose of this Phantom study was to evaluate the effect of the size of the lesion and SBR on the threshold to be used for PET tumor volume delineation. Materials and Methods: Phantom used in the study comprised a sphere–cylinder assembly containing six spheres of different inner diameters (1.10, 1.35, 1.44, 1.50, 1.83 and 1.93 cm) with inner volumes of 0.70, 1.30, 1.50, 1.77, 3.22 and 3.82 cm3, respectively. The scans were acquired with SBR of 6:01, 7:01, 8:01 and 10:01. These SBRs were calculated from 42 patients with lymphoma to simulate clinical images. PET tumor volume was calculated using RT_Image software at different threshold values (40, 45, 50, 55, 60, 65, 70 and 75% of SUVmax) for each sphere at different SBRs. The threshold intensity value at which the calculated volume was nearly equal to actual volume of spheres was considered as the standardized threshold intensity (STI) value. Results: STI values depended on the diameter of the sphere and not on the SBR. It is found that 40% threshold is suitable for calculating the volume of any lesion with diameter greater than 1.83 cm, 60% for diameter greater than 1.35 cm but less than 1.83 cm, and 75% for diameter less than 1.35 cm. Conclusion: Size-dependent thresholding is an accurate and reproducible method of tumor volume delineation on PET-computed tomography (CT).