C–reactive protein (CRP) is a phylogenetically highly conserved plasma protein that participate in the systemic response to inflammation. Its plasma concentration increases during inflammatory states, a characteristic that has long been employed for clinical purposes. CRP is a sensitive marker of systemic inflammation. CRP is believed to be both a marker and a mediator of atherosclerosis and Coronary Heart Diseases (CHD). In the present study, the CRP levels were studied in 122 patients of Rheumatoid Arthritis and 120 patients of Coronary Heart Diseases. The levels more than 0.6mg/dl were marked to be higher. For the detection of CRP in serum semi–quantitative method was used. It was found that CRP level is higher in age group B than age group A in RA and CRP level is higher in age group A than age group B in CHD. This shows that CRP level can differ according to age differences.