Kusta Gaja Kesari is a herbo-mineral Siddha formulation consisting of abraga parpam, kantha chenduram, Aya chenduram and rasa parpam which has the indication for skin diseases.The aim of the study is to evaluate the anti-bacterial and anti-fungal activity of Kusta Gaja Kesari. The Anti-bacterial and Anti-fungal activity of Kusta Gaja Kesari is evaluated by determining minimum inhibitory concentration (MIC) using resazurin microtitre assay. It was observed that the Kusta Gaja Kesari reveals significant activity against the pathogen E-coli (MIC 250µg) and Staphylococcus aureus (MIC 250µg) with the lowest MIC values when compared to that of Staphylococcus aureus (MIC 500µg) and Pseudomonas aeruginosa with MIC value of 500µg. Further the sample found less effective against Streptococcus pyogenes which is provoked with highest MIC value of about 1000 µg. Similarly, the samples demonstrate consistent anti-fungal activity against Aspergillus Niger and Candida albicans – CA with the MIC value of 250 and 500µg respectively. The present investigation shows that the Siddha Herbo mineral formulation Kusta Gaja Kesari reveals convincing Anti-bacterial and Anti-fungal activity among all tested organisms.