This study aims to evaluate the impact of clinicopathological factors on metabolic parameters of 18F-FDG PET/CT in the different staging of breast cancer. 15 histopathologically confirmed breast cancer lesions on patients who underwent whole- body FDG-PET/CT for staging were retrospectively reviewed. Three PET/CT metabolic parameters including semiquantitative maximum standardised uptake value (SUVmax), metabolic tumour volume (MTV) and total lesion glycolysis (TLG) were quantified over the primary lesions of the breast. The parameters were statistically correlated with clinicopathological factors including patients’ age, body mass index (BMI) and tumour size. Multivariate regression was performed to determine the significant factors that best predicted those metabolic parameters. Tumour size was significantly correlated to all metabolic parameters (p < 0.001) and was a significant predictor of those parameters (p < 0.001). Tumour size accounted for 77.5%, 94.3% and 96.2% of the SUVmax, MTV and TLG variances, respectively. Tumour size significantly affects the metabolic parameters of 18F-FDG PET/CT (SUVmax, MTV and TLG) in the staging of breast cancer. Therefore, tumour size has a prognostic value for the staging of breast cancer in a 18F-FDG PET/CT study for improved clinical management.