Abstract： Objective To investigate the diagnostic value of ADC in the evaluation of small round cell malignant tumors(SRCMT) of nasal and paranasal sinus.Methods This study included 143 patients with surgically confirmed SRCMT and Non-SRCMT of nasal and paranasal sinus between 2008 and 2015, all patients underwent diffusion weighted MRI at 3.0 T with a b factor of 0 and 1 000 s/mm2.Quantitative analysis of ADC values was performed.Difference in ADC values between SRCMT and Non-SRCMT was evaluated using the independent samples t test.One-way analysis of variance(ANOVA) test was performed to compare the ADC values of SRCMT.Receiver operating curves (ROC) were developed to determine the cutoff points to differentiate SRCMT from Non-SRCMT.Results There were 98 SRCMT, of which 20 lesions were rhabdomyosarcoma(RMS), 19 lesions were non-Hodgkin's lymphoma(NHL), 4 lesions were malignant melanoma(MM), 14 lesions were neuroendocrine carcinoma(NEC), 12 lesions were Ewing sarcoma or primitive neuroectodermal tumor(EWS or PNET), 11 lesions were extramedullary plasmacytoma(EMP), and 8 lesions were olfactory neuroblastoma(ON).There were 45 Non-SRCMT, of which 28 lesions were squamous cell carcinoma(SCC) and 17 lesions were adenoid cystic carcinoma(ACC).The mean ADC value of SRCMT[(0.66 ± 0.12) × 10-3mm2/s] was significantly different (t=14.97, P＜0.01) from Non-SRCMT [(1.02± 0.16)× 10-3mm2/s].All of 7 kinds of SRCMT were divided into 3 groups according to ADC values: NHL,MM, NEC,EMP;RMS,EWS,PNET;ON.There was statistically significant difference among all 3 groups(F=39.743, P＜0.01), and the differences between any 2 groups were still statistically significant.The area under the ROC of ADC values diagnosing SRCMT was 0.975.Compared with pathological results, an ADC value of 0.82 × 10-3mm2/s was used as the threshold for diagnosing SRCMT with a sensitivity of 97.8％ (44/45),specificity of 89.8％(88/98), and accuracy of 92.3％ (132/143).ADC value had high correlations compared with pathological results (Kappa value was 0.831).Conclusion The ADC value is a non-invasive imaging parameter that can be used to effectively assess SRCMT of nasal and paranasal sinus.