Several different models for the signal decay in diffusion weighted imaging (DWI) have been proposed. These include the classical monoexponential model with the apparent diffusion coefficient as free parameter and the quite popular intravoxel incoherent motion (IVIM) model, which includes perfusion effects present at low b-values^[1]. In this study we compared the association of IVIM derived parameters with the histologic grade, N-stage, EGFR expression and K-RAS gene mutation of primary rectal cancer.This retrospective study was institutional review board approved, and informed consent was waived. 35 patients with rectal adenocarcinoma confirmed by pathology underwent MR imaging before surgery. Research sequences included diffusion-weighted magnetic resonance (MR) imaging with 15 b values (10，20，30，40，60，80，100，150，200，400，800，1000，1200，1500，2000sec/mm²). In all patients, surgery was performed without neoadjuvant therapy. The region-of-interest (ROI) was selected based on the b1000 image, avoiding the area of cystic and necrosis. Pure molecular diffusion (D), perfusion-related diffusion (D*) and perfusion fraction (f) were calculated. The average value of each parameter in different ROIs was recorded. The differences between the histopathologic parameters were assessed by using the Chi-square test, including t-test (for normally distributed data) and Rank sum test (for non-normally distributed data). The histopathologic parameters included A (differentiation grade: A1, well differentiated; A2, moderately differentiated; A3, poorly differentiated), B (N-stage: B1, lymph nodes involvement; B2, lymph nodes without involvement), C (EGFR expression: C1, positive; C2, negative) and D (K-RAS gene mutation: D1, positive; D2, negative).The data of the D values was non-normally distributed, While the data of both the D* and f values were normally distributed. Mean D values of the well differentiated group (A1) , moderately differentiated group (A2) and poorly differentiated group (A3) were（1.133±0.451）×10^-3mm²/s、（0.690±0.373）×10^-3mm²/s and（0.519±0.253）×10^-3mm²/s respectively. Mean D values of A1 were higher than that of A2 (Z=-2.283, p=0.011, p<0.05 statistically significant) and A3 (Z=-2.417, p=0.0075), whereas there was no significant difference in mean D values between A2 and A3 (Z=-1.195, p=0.119). The ROI region selection referent to Figure.1. Mean D* values of B1 and B2 were（40.523±17.288）×10^-3mm²/s and（65.100±12.602）×10^-3mm²/s respectively. There was a significant difference in mean D* values between B1 and B2 (t=4.553, p=0.000). Mean f values of B1 and B2 were（0.491±0.172）and（0.385±0.139）respectively. There was a significant difference in mean f values between B1 and B2 (t=-1.835, p=0.042). There were no significant differences in D, D* or f values when stratifying patients according to EGFR expression and K-RAS gene mutation. The rest of the specific data was recorded in Table.1-4.Significant correlations were found between D values and differentiation grade, D* values and N-stage, f values and N-stage. IVIM–derived parameters may be a promising imaging biomarker of tumor aggressiveness and prognosis.