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Purpose: Pre- and post-treatment MRI based Diffusion Weighted Imaging (DWI) has shown to be an effective predictor and indicator of treatment response in cancer. However, clinical applications of the Apparent Diffusion Coefficient (ADC) requires sufficient precision; this can be assessed with repeatability studies. Therefore, the purpose of this study was to assess the repeatability of ADC measurements on an MR-Linac for patients with locally advanced rectal cancer (LARC). Additionally, the relative change in ADC during treatment was compared against the repeatability. Methods: For 17 patients, DWI was performed once on 3T MRI during pre-treatment, and twice on an 1.5T MR-Linac during each treatment fraction. Manual delineations of the Gross Tumour Volume (GTV) were created by the author. In addition two semi-automatic delineation methods were implemented, which used; a registration pipeline to propagate T2 delineations; a geometric distortion correction algorithm to correct for DWI susceptibility artefacts. Based on visual inspection the most accurate and consistent delineations were used to calculate the ADC; Bland-Altman repeatability coefficient (RC), within subject coefficient of variation (wCV), and the intraclass correlation coefficient (ICC). Lastly, the relative change in mean and median tumour ADC was compared against the RC. Results: Manual delineations were determined to have the highest agreement with ADC tumour location and were used in subsequent calculations. The mean and median tumour ADC wCV was 7.6% (CI95:5.5-9.2%) and 8.2% (CI95:5.9-9.9%) respectively, which corresponds with a RC of 21.0% (CI95:15.1-25.6%) and 22.7% (CI95:16.3-27.4%). The reliability of the measured data was good, with an ICC of 0.81 (CI95:0.72-0.87). In 6 out of 17 patients the relative change in ADC exceeded the RC at some point during treatment, which suggest a potential for future clinical applications. Conclusion: Despite the low precision of rectal cancer ADC me, Biomedical Engineering | Medical Physics