Background: To analyze the longitudinal length accuracy of gross tumor volume (GTV) delineation with diffusion weighted magnetic resonance imaging for esophageal squamous cell carcinoma (SCC). Methods: Forty-two patients from December 2011 to June 2012 with esophageal SCC who underwent radical surgery were analyzed. Routine computed tomography (CT) scan, T2-weighted MRI and diffusion weighted magnetic resonance imaging (DWI) were employed before surgery. Diffusion-sensitive gradient b values were taken at 400, 600, and 800 s/mm(2). Gross tumor volumes (GTV) were delineated using CT, T2-weighted MRI and DWI on different b-value images. GTV longitude length measured using the imaging modalities listed above was compared with pathologic lesion length to determine the most accurate imaging modality. CMS Xio radiotherapy planning system was used to fuse DWI scans and CT images to investigate the possibility of delineating GTV on fused images. Results: The differences between the GTV length according to CT, T2-weighted MRI and pathology were 3.63 +/- 12.06 mm and 3.46 +/- 11.41 mm, respectively. When the diffusion-sensitive gradient b-value was 400, 600, and 800 s/mm(2), the differences between the GTV length using DWI and pathology were 0.73 +/- 6.09 mm, -0.54 +/- 6.03 mm and -1.58 +/- 5.71 mm, respectively. DWI scans and CT images were fused accurately using the radiotherapy planning system. GTV margins were depicted clearly on fused images. Conclusions: DWI displays esophageal SCC lengths most precisely when compared with CT or regular MRI. DWI scans fused with CT images can be used to improve accuracy to delineate GTV in esophageal SCC.