Objective: To evaluate the value of the apparent diffusion coefficient (ADC) in predicting treatment response in rectal cancer patients after neoadjuvant chemoradiation therapy (nRCT). Subject and method: A study was conducted on 29 rectal cancer patients who underwent nRCT. Patients underwent magnetic resonance imaging (MRI), and tumor volume and ADC values (b-values: 0-1000s/mm²) were measured before and after nRCT. ADC values were measured in two ways: 1. For the entire tumor (ADC1) and 2. On the slice with the largest tumor area (ADC2). Changes in tumor volume (ΔV) and ADC values (ΔADC) before and after treatment were compared with the surgical pathology results, categorized into complete response (pCR)/incomplete response and downstaging T/not downstaging T groups. Result: ΔADC values were higher in the downstaging T group compared to the non-downstaging T group, with ΔADC1 values of 45.4% and 19.1%, respectively (p = 0.005), and ΔADC2 values of 51.7% and 20.2%, respectively (p = 0.01). At the ΔADC1 threshold of 42.2%, sensitivity was 55.6%, and specificity was 100%, while for ΔADC2 at 18.1%, sensitivity and specificity were 88.9% and 72.7%, respectively. There was no statistically significant difference in ΔADC values between the complete response and incomplete response groups (p>0.05). Changes in tumor volume did not provide prognostic value for treatment response to NRCT. Conclusion: The ΔADC value is a potential prognostic factor in predicting treatment response in rectal cancer patients after nRCT, with more accurate diagnostic information compared to tumor volume.