BACKGROUND: To investigate the feasibility of using 18F-fibroblast activation protein (FAP) inhibitor positron emission tomography/computed tomography (18F-FAPI PET/CT) to detect intestinal fibrosis in its early stages and identify its severity in a mouse model. METHODS: A dextran sulfate sodium (DSS)-induced mouse model of intestinal fibrosis was established. To detect pro-inflammatory cytokines and histopathology, blood and intestinal lesion samples were collected after 18F-FAPI PET/CT scanning (3.7 MBq/mice) at 3, 6, 9, and 12 weeks post-initial exposure to DSS. Correlation and diagnostic efficacy were explored between 18F-FAPI uptake and FAP expression or fibrosis score in early, late, and entire stages of intestinal fibrosis. RESULTS: 18F-FAPI uptake was positively correlated with FAP expression throughout entire stages of intestinal fibrosis (r = 0.90). However, a weak correlation between 18F-FAPI uptake and fibrosis score (r = 0.49), and moderate diagnostic performance of 18F-FAPI PET for fibrotic severity (area under the receiver-operating characteristic curve [AUC] = 0.79) were found throughout the entire stages. Interestingly, in the early stages, 18F-FAPI PET effectively distinguished the degree of intestinal fibrosis (AUC = 0.95), and was strongly correlated with fibrosis score (r = 0.89). In the late stages, the diagnostic efficacy (AUC = 0.46) and correlation (r = -0.20) drastically decreased. CONCLUSIONS: As Crohn's disease (CD) with intestinal fibrosis progresses, 18F-FAPI uptake is high in the early stages and then gradually decreases. Activated fibroblasts appear more frequently in the early stages of intestinal fibrosis indicates that 18F-FAPI PET has a great potential for early identification of intestinal fibrosis and provides new insights into treatment decision-making in CD patients.