System suitability test (SST) is a test to determine the suitability and effectiveness of chromatographic system prior to use. The performance of any chromatographic system may continuously change during their regular use, which can affect the reliability of the analytical results. The operation parameters of the whole chromatographic system can be checked with properly selected SST mixtures. These mixtures are used to establish characteristic chromatographic parameters, such as the number of effective theoretical plates, resolution, asymmetry, detection limit and selectivity. The system is then only declared suitable if the responses are within given limits. This study was performed to set system suitability criteria for detectability in gas chromatography-nitrogen phosphorus detection (GC-NPD) methods for pesticide residue analysis. Under our laboratory and analysis conditions, asymmetry and tailing of dimethoate and tributylphosphate (TBP) peaks slightly changed after 63 standard and/or sample injections; however these changes were within the required limits. Key Words: System Suitability Test (SST), GC Performance, GC Optimization.