Blood is the life in our vessels. It makes up the 8% of our body weight. Loss of blood from 750 ml to 2000 ml is enough to enter us to shock condition due to loss of circulating volume. In situations like these, urgent blood transfusion needs to be handled with extensive care. Not all emergency services may have haematology experts, a good helper application with blood transfusion rules that also determines the possible donors for each patient is necessary. In this paper, we present blood ontology that is an extension of FOAF ontology to serve as a core service of blood transfusion process. We had researched the possible blood transfusion rules of different health organization all over the world and highlight the blood transfusion rules. These rules are embedded to a ontology structure includes blood, FOAF and Relationship ontologies. This ontology structure not only serves as storage for patient and donor information, but also as a knowledge extraction structure to determine the possible donors and help doctors in treatment.