Background/Aims: The objective of this study is to measure mean platelet volume (MPV) and platelet cluster of differentiation molecule 40 ligand (CD40L) to evaluate their usefulness in the diagnosis of spontaneous bacterial peritonitis (SBP) in cirrhotic patients. Materials and Methods: This study comprised 80 cirrhotic patients with ascites. MPV and platelet CD40L were measured Results: A significant increase in MPV and platelet CD40L observed in SBP group compared to non SBP (P < 0.001). At cutoff value of 8.71 fl MPV had 68.75% sensitivity and 80% specificity for diagnosis of SBP. At cutoff value of 4,81 ng/ml platelet CD40L in serum had 62% sensitivity and 80% specificity for detecting SBP. Conclusion: mean platelet volume (MPV) and platelet cluster of differentiation molecule 40 ligand (CD40L) are useful marker in the diagnosis of SBP in cirrhotic patients.