The relationship between 18F-FDG uptake and HER2 expression in colorectal cancer has not been investigated yet. This study aimed to investigate the predictive efficiency of preoperative 18F-FDG PET/CT for HER2 expression and prognosis in colorectal cancer. We retrospectively analyzed 131 colorectal cancer patients who underwent 18F-FDG PET/CT scans in our center before surgery. HER2 positivity was defined as a score of 2+ or 3+, and HER2 negativity was defined as a score of 0 or 1+ in immunohistochemistry of HER2 expression. The relationships between 18F-FDG PET/CT metabolic parameters and HER2 expression and the prognosis of colorectal patients were systematically studied. From 131 colorectal cancer patients, there were 27 (20.6%) HER2-positive patients. SUVmax of the primary tumor (mean ± SD) in the HER2-positive and the HER2-negative group was 18.238±8.912 and 14.455±6.531, respectively. SUVmax in the HER2-positive group was higher than in the negative group (p=0.034). When the cutoff was based on 5 cm, tumor size demonstrated significant positive correlations with SUVmax (p=0.012) and HER2 expression (p=0.014). Multivariate analysis showed that both SUVmax and tumor size had a significant correlation with HER2 expression (p=0.049 vs. p=0.043, respectively). There was no statistical difference in PFS between the HER2-positive and the HER2-negative group (p=0.28). 18F-FDG metabolic parameters had a significant correlation with HER2 expression in colorectal cancer. SUVmax combined with primary tumor size were better for predicting the HER2 status of colorectal cancer. 18F-FDG metabolic parameters had a significant correlation with HER2 expression in colorectal cancer. SUVmax combined with primary tumor size were better for predicting the HER2 status of colorectal cancer.