Background: This study aimed to select the optimum computerized (CT) slice thickness by analyzing its effect on the volumes and dosimetric parameters in treatment planning of low grade Glioma. Material and Methods: Fused brain CT and magnetic resonance imaging (MRI) images of 17 patients were used for treatment planning for three-dimensional conformal radiation therapy (3D-CRT) and Intensity-modulated radiation therapy (IMRT) using CT dataset with the thickness of 1.5 mm, 3, 5, and 10 mm slice. Volume of target and organs at risk (OARs), dosimetric parameters and contouring times using the reconstructed images were compared with the original dataset. Results: Using larger than 3 mm slice thicknesses resulted in significant increase up to 76% in target volumes as well as the volumes of OARs (p-value<0.05). The variation in normalization point coordinates was also significant using larger than 3 mm slice thicknesses. The contouring time of tumor and OARs using the original data set was up to 35% more compared to the 3 mm dataset. Dosimetric parameters were comparable for 1.5 and 3 mm datasets, however, significant difference of up to 200% was observed for 5 and 10 mm datasets (P<0.05). Conclusions: CT slices larger than 3mm resulted in significant inaccuracies in volumes and dose coverage of target and OARs. Although using the slice thickness larger than 1.5 mm reduced contouring time significantly, this slice thickness is only recommended when the oncologist is not concerned about the dose received by the small critical organs especially those adjacent to the tumor.