Objective To establish the optimal morphological criteria combined with fibrinogen level for evaluation of lymph node metastasis in colorectal cancer. Methods A consecutive series of 690 patients who underwent curative surgery for colorectal cancer, were examined by abdominopelvic enhanced multi-slice spiral computed tomography (MSCT) scan. If regional lymph nodes appeared, the maximal long-axis diameter (MLAD), maximal short-axis diameter (MSAD), and axial ratio (MSAD/MLAD) were recorded. At each lymph node size cut-off value, the following were calculated: accuracy, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). Moreover, preoperative plasma level of fibrinogen was retrospectively examined to identify metastatic or inflammatory lymph node combined with MSCT image. Both modalities, MSCT plus fibrinogen and MSCT alone, were compared based on the pathologic findings. Results The study population consisted of 100 patients with regional lymph nodes show. No significant difference was found between metastatic and inflammatory lymph nodes in imaging characteristics (Pgt;0.05). The best cut-off value of MSAD was 6 mm for lymph node metastasis with the sensitivity of 46.8%, specificity of 68.4%, accuracy of 55.0%, PPV of 70.7% and NPV of 44.1%. The best cut-off value of MLAD was 8 mm with the sensitivity of 43.5%, specificity of 63.2%, accuracy of 51.0%, PPV of 65.9% and NPV of 40.7%. Using hyperfibrinogenemia (FIB ≥3.5 g/L) to identify small metastatic lymph node, of which MSAD lt;6 mm or MLAD lt;8 mm, showed statistical diagnostic value (Kappa=0.256, P=0.047). Compared with MSAD (6 mm) alone, MSAD (6 mm) combined with hyperfibrinogenemia had a higher sensitivity (79.0% vs. 46.8%, Plt;0.001), but a similar accuracy (66.0% vs. 55.0%, Pgt;0.05) and a lower specificity (44.7% vs. 68.4%, P=0.037). MLAD (8 mm) combined with hyperfibrinogenemia led to a greater diagnostic value in sensitivity (80.6% vs. 43.5%, Plt;0.001) and accuracy (66.0% vs. 51.0%, P=0.031) than MLAD (8 mm) alone, with a no-significantly decreasing specificity (42.1% vs. 63.2%, Pgt;0.05). Conclusions This present study recommend MSAD ≥6 mm or MLAD ≥8 mm as the optimal criteria for preoperative N staging in colorectal cancer. Moreover, the sensitivity and even accuracy could be improved by combining hyperfibrinogenemia for lymph node metastasis identification.