Objectives To compare the intravoxel incoherent motion (IVIM) diffusion and perfusion characteristics of nasopharyngeal carcinoma (NPC) and post-chemoradiation fibrosis to aid in their differentiation. Methods Fifty-three (64 %) patients with newly diagnosed NPC and 30 (36 %) patients with biopsy-proven post-chemoradiation fibrosis were recruited into tumour and fibrosis groups respectively. Diffusion-weighted magnetic resonance (MR) imaging was performed using 13 b values (0–1,000 s/mm 2 ). Their respective IVIM parameters ( D , pure diffusion; f , perfusion fraction; D *, pseudodiffusion coefficient) were obtained. Results D and f were significantly lower in NPC ( D  = 0.752 ± 0.194 × 10 -3  mm 2 /s, P <0.001; f  = 0.122 ± 0.095, P <0.001) than in fibrosis ( D  = 1.423 ± 0.364 × 10 -3  mm 2 /s; f  = 0.190 ± 0.120); while D * was significantly higher in NPC (111.366 ± 65.528 × 10 -3  mm 2 /s, P <0.001) than in fibrosis (77.468 ± 62.168 × 10 -3  mm 2 /s). Respective cut-off values with sensitivity, specificity and accuracy were: D  = 1.062 × 10 -3  mm 2 /s (100 %, 100 %, 100 %); f  = 0.132 (66.0 %, 100 %, 78.3 %); D * = 85.283 × 10 -3  mm 2 /s (100 %, 90.7 %, 96.4 %). Conclusion NPC and post-chemoradiation fibrosis have distinctive IVIM parameters. IVIM MR imaging is potentially useful in discrimination between NPC and fibrosis. Key Points • New MRI techniques offer greater help in the assessment of nasopharyngeal carcinoma . • Tumour and post - chemoradiation fibrosis have distinctive intravoxel incoherent motion diffusion / perfusion parameters . • Non - invasive IVIM MRI may help differentiate between tumour and fibrosis . • Pure diffusion is a robust independent discriminating factor which improves diagnostic confidence .