OBJECTIVE To explore the change of contrast sensitivity (CS) in diabetic patients without diabetic retinopathy. METHODS It was a case-control study. We screened and tested 52 type 2 diabetic patients (89 eyes) and 47 healthy persons (68 eyes). OPTEC 6500 contrast sensitivity tester was used to examine near and distance CS values on 1.5, 3.0, 6.0, 12.0, 18.0 cycles/degree (c/d) respectively. We kept strict inclusion criteria on subjects, no retinopathy was found and best corrected vision acuity was above 1.0. The difference of CS values between these two groups was investigated, and the influence of gender, age, duration of diabetic mellitus and application time of insulin on CS values, respectively. T-test or u-test was used to compare the means of CS values between these two groups, near and distance CS values in each group. Simple linear correlation and multiple regression analysis were used to analyze the single factor and multiple factors correlation between CS values and age, duration, gender, and application time of insulin, respectively. RESULTS The maximum CS values in both distance and near were 112.67 ± 43.97, 85.85 ± 41.83 and 83.68 ± 35.91, 55.20 ± 24.14, both appeared in 6.0 c/d. The minimum CS values in both distance and near were 26.19 ± 17.69, 12.69 ± 7.09 and 18.68 ± 11.81, 8.22 ± 5.68, respectively, both appeared in 18.0 c/d. Distance and near CS values in diabetic group and control group had very significant differences at these five spatial frequencies (From 1.5 to 18.0 c/d, the test values on distance, t = 3.11, 3.38, 2.77, 3.48, 3.86; P < 0.05; the test values on near, u = 3.70, 3.22; t' = 3.69, 3.88, 4.25; P < 0.05). Abnormal rate in diabetic group on high, medium, low and all frequencies were higher than control group, regardless of distance or near CS values (in near χ(2) = 11.86, 8.17, 9.14, 5.81; P < 0.05; on 4 combinations in distance χ(2) = 13.27, 6.70, 4.01, 4.50; P < 0.05). In diabetic group, gender related with 3.0 and 6.0 c/d CS values on near-distance (r = 0.26, 0.28; P < 0.05); age related with high-frequencies on near (r = -0.45, -0.28; P < 0.05); duration of diabetic mellitus related with high-frequencies on near (r = -0.25, -0.39; P < 0.05), and related with medium, high frequencies on distance (r = -0.26, -0.28, -0.30, -0.34; P < 0.05); application time of insulin related with high-frequencies on distance (r = -0.30, -0.31; P < 0.05). CONCLUSIONS There exists important sense of CS value on showing visual function changes accurately in early-stage of diabetes mellitus. The high frequencies on distance is the sensitive index in vision impairment. Duration of diabetes closely relates with changes of retinal function ultimately, can be classified as the most important risk factor of CS values decline in early-stage diabetic patients.