Background: Spontaneous bacterial peritonitis (SBP) is a severe and frequent complication of cirrhosis with a high mortality rate. Early diagnosis and treatment of SBP is necessary for survival. Objective: To study the mean platelet volume (MPV) versus leucocyte esterase as a marker in diagnosis of decompensated cirrhotic patients with spontaneous bacterial peritonitis. Patients and methods: A total of 200 patients with decompensated liver cirrhosis and ascites, were admitted to the Department of Gastroenterology and Hepatology in Nasser Institute, and were enrolled in this study between January 2016 and October 2017. Patients were divided into two equal groups; group (I) was diagnosed non SBP and group (II) was diagnosed SBP. All patients underwent abdominal paracentesis, and the ascitic fluid was processed for cell count, leukocyte esterase reagent strip test (LERS) (URIT 10V) and culture. All patients underwent assessment of MPV in CBC. Results: Diagnostic performance of mean platelet volume showed that we can use MPV as a good diagnostic marker for SBP with sensitivity 75 %, specificity 99% and accuracy 93.2 %. The best cut off value for discriminating patients with SBP from patients without SBP was 9.2 fl. The test of ascetic fluid by leukocyte esterase reagent strips showed that specificity = 93% , sensitivity = 80%, positive predictive value (PPV) = 92%, negative predictive value (NPV) =82.3% and accuracy = 86.5%. Conclusion: MPV and LERS can be used as a good marker in diagnosis of SBP in cirrhotic patients with ascites.