Abstract： Objective To determine the utility of diffusion-weighted magnetic resonance imaging ( MR DWI ) in detecting tumor necrosis with histological correlation after neoacljuvant chemotherapy. Methods Conventional MRI and DWI were obtained from 36 patients with histological proven esteosarcoma. Magnetic resonance examinations were performed in all patients before and after 4 cycles of preoperative neoadjuvant chemotherapy. Apparent diffusion coefficients (ADC) were calculated. The degree of tumor necrosis was assessed using the histological Huvos classification after chemotherapy. T-test was performed for testing changes in ADC value between the 2 groups. P value less than 0. 05 were considered as a statistically significant difference. Results The differences in ADC between viable [ (1.06±0. 30) ×10-3mm2/s ] and necrotic [ (2. 39±0. 44 )×10-3mm2/s] tumor were significant (t= 3. 515,P<0. 05). Changes in ADC value was greater in good responses to neoadjuvant chemotherapy than in poor responses, the ADC value in good responses was increased from (1.18±0. 19)×10-3mm2/s to (2. 27±0. 20)×10-3mm2/s, the corresponding value in poor responses was increased from (1.45± 0.11)×10-3mm2/s to (1.83±0. 16)×10-3mm2/s, There was significant difference in changes of ADC values between good responses and poor responses ( t = 4. 981, P < 0. 01 ). Conclusion Diffusion-weighted MRI permits recognition of tumor necrosis induced by chemotherapy in osteosarcoma. DWI is correlated directly with tumor necrosis. They have potential utility in evaluating the preoperative chemotherapy response in patients with primary osteosarcoma.