The skill of supra-12th rib mini-flank approach for open partial nephrectomy (MI-OPN) provides an advanced operative method for renal tumor. Compared with laparoscopic and robotic surgery, it may be a feasible selection for the complex renal tumors. We describe our techniques and results of MI-OPN in complex renal tumors with high RENAL nephrometry score (RENAL nephrometry score ≥10). Fifty-five patients diagnosed with renal tumors between January 2009 and July 2013 were included in this study. Eligibility criteria comprised of patients with complex renal tumor (RENAL score ≥10) being candidates for partial nephrectomy (PN). All patients received MI-OPN and all surgeries were performed by a single urologist. The preoperative workup comprised of medical history, physical examination, and routine laboratory tests. Serum creatinine was recorded preoperatively and 2 to 3 months after operation. Operative time, ischemia time, blood loss, operative and postoperative complications, renal function, and pathology parameters were recorded. MI-OPN was successfully performed in all cases. Mean tumor size was 4.7 cm (range: 2.5-8.1). Mean warm ischemia time was 28.1 minutes (range: 21-39), mean operative time was 105 minutes (range: 70-150) and mean estimated blood loss was 68 mL (range: 10-400). Mean postoperative hospital stay was 6.5 days (range: 5-12). Postoperative complications were found in 3 patients (5.5%). The mean pre- and postoperative serum creatinine levels were 76.2 μmol/L (range: 47-132) and 87.1 μmol/L (range: 61-189) with significant difference (P = 0.004). The mean pre- and postoperative estimated glomerular filtration rate (eGFR) were 91.5 (range: 34-133) and 82.5 (range: 22-126.5), respectively with significant difference (P = 0.024). In an average follow-up of 19.9 months (range: 8-50), no local recurrence or systemic progression occurred. In conclusion, MI-OPN can combine the benefits of both minimal invasive and traditional open partial nephrectomy (OPN) techniques with a smaller incision. It is an innovation of traditional OPN and suitable for the complex renal tumors with high RENAL nephrometry score safely and effectively.