Senna siamea is an important medicinal plant having widespread application in a variety of disorders. The aim of this study was the phytochemical investigation and evaluation of Cytotoxicity, Analgesic Antidiarrhoeal, Antimicrobial activities of the leaf of Senna siamea. Phytochemical screening of the ethanolic extract of Senna siamea ensured the presence of glycosides,tannins and steoids. In cytotoxicity activity test the crude extracts of leaves were found to show less activity.The crude ethanolic extract exhibited dose dependent analgesic activity. The ethanol extract of Senna siamea at the dose of 500 mg/kg exhibit significant (P< 0.001) inhibition of writhing reflex by 61.98% while the standard drug diclofenac Na inhibition was found to be 85.95 % at a dose of 25 mg/kg body weight. In the castor oil-induced diarrhoeal mice, the ethanol extract of Senna siamea at the dose of 500 mg/kg, reduced the total number of faeces as well as of diarrhoeic faeces, and the result was statistically significant In anti-microbial activity test by disc-diffusion method the extract showed no activity against the test organisms at the dose of 500µg/disc in comparison with standard drug Kanamycin.