Introduction & Purpose: Percutaneous nephrolithotomy (PCNL) is a well-proven minimally invasive treatment modality for the management of renal calculi. However, PCNL is not without complications. This study aims to assess the complications after PCNL using the Modified Clavien grading system and the success rates of PCNL using Guy's Stone Score (GSS). Methods: 100 patients were enrolled in this study at Meenakshi Mission Hospital Madurai Tamilnadu, India, in duration April 2020 to March 2022. The stone burden and predictive difficulty were determined by radiographic studies and classified using the GSS. Patient underwent PCNL as per the standard protocol after ensuring sterile urine. The modified Clavien grading system was used for evaluating perioperative and postoperative complications of PCNL. Results: All renal units with GSS I were stone-free after one session of PCNL, GSS II needed one session in 83.3% and two sessions in 11.1% renal units, GSS III needed one session in 45.5% and two sessions in 36.4% renal units and GSS IV needed one session in 20% and two sessions in 40% renal units and remaining 40% renal units could not be made stone free in spite of multiple sessions. Total 87 complications occurred in 41 patients, grade 1 to 5 includes 28 (32.3%), 38 (43.7%), 7 (8%), 7 (8%), 3 (3.4%), 3 (3.4%), 1(1.1%). Conclusion: PCNL is standard care for large renal calculus. It is a safe, efficient, and feasible technique. Guy’s score is a very simple and effective system to classify the complexity of various renal stones. It can guide urologists and, patients to make their decision and consent about different aspects of surgery. The Modified Clavien–Dindo system is easy to use and feasible tool to grade perioperative complications.