Twenty one broiler chickens were divided into three treatments of Spirulina sp, i.e: P0:0% Spirulina sp, P1:10% Spirulina sp, P2:20% Spirulina sp of the fresh water algae, each of which consisted of seven replicates, in drinking water. The treatment of Spirulina was given to broiler from the age of 7 days old to 32 days old. Artificial infection of Avian Influenza virus H5N1 (A/Ck/Indonesia/BL/03 from isolate collection of Adi Prijo Rahardjo M.Si., DVM) was given to 26 day old chicken by entering the respiratory tract (nose drops) using a dose of 0.1 inoculum containing 107 EID50. Inoculated chicken were observed for 6 days after inoculation, during which the clinical signs were recorded. Blood samples were collected from brachialis vein for as much as 0.5 – 1 ml in tube containing EDTA (1 mg/tube) to calculate leukocyte cell. Dead chickens in this research were necropsied for the determination and heart tissues of chicken were collected for histopathologic and immunohistochemistry examination. Heart tissues were fixed by submersion in 10% neutral buffered formalin, routinely processed, and embedded in paraffin. Sections were made at 7 μm and were stained with hematoxylin and eosin (HE) and a duplicate 7 μm section was conducted immunohistochemically with primer antibody (anti H5N1