Objective: Key hemorheological variables are associated with several life-threatening diseases including cardio-cerebro-vascular diseases. A diet can influence the blood rheological variables. To compare the effectiveness of a vegetable soup on blood viscosity (BV), hematocrit (Hct), plasma fibrinogen, lipid profile, fasting blood sugar (FBS), and blood osmolarity in patients with polycythemia in comparison to a control group. Materials and Methods: This randomized controlled trial study was conducted at Isar health clinics in Mashhad, Iran, during a 7-month period. Forty male participants (35 to 60 years old) with polycythemia, but without underlying diseases, were included. They randomly assigned to two groups and either received diet/phlebotomy or phlebotomy alone, for 6 weeks. The data were analyzed by SPSS version 16 using parametric tests. Results: A significant reduction in BV at 30s (p≤ 0. 001), BV at 40s (p≤ 0. 001), BV at 50s (p≤ 0. 001), Hct (p≤ 0. 001), plasma fibrinogen (p≤ 0. 001), total cholesterol (p<0. 01), LDL-cholesterol (p<0. 01), VLDL-cholesterol (p≤ 0. 001), HDL-cholesterol (p≤ 0. 01), osmolarity (p≤ 0. 001), and FBS (p≤ 0. 001) was observed in diet recipients. Following the intervention, there was a significant decrease in triglyceride (intervention group, p<0. 05; control group, p<0. 05), in both groups. Conclusion: This trial showed that the plant– based food used in this study could improve blood rheology.