Congenital adrenal hyperplasia (CAH) is an inherited autosomal recessive disease which results in steroid hormone metabolism disorders. Steroid 11beta-hydroxylase deficiency due to mutations in the CYP11B1 gene is the second most common cause of CAH while CYP21 deflection is the first one. Mutations in the CYP11B1 gene directly affect the activity of the enzyme 11beta-hydroxylase. The enzyme 11beta-hydroxylase deficiency leads to accumulation of intermediate precursor. In this study, the authors used PCR and sequenced directly to identify mutation. CYP11B1 gene of patient including 9 exons and 8 introns were sequenced and compared with normal CYP11B1 gene. Missense mutation of 11beta-hydroxylase was expessed in COS-1 cells to study the effect of mutant on the conversion of 11-deoxycortisol to cortisol. A novel missense mutation p.R51K was detected in CYP11B1 which causes a decrease of 29 +/- 4.5 percent of lip-hydroxylase activity as compare to wild type. The patient with clinical manifestations, biochemical disease of CAR classical forms such as external genital abnormalities, reduced cortisol and Na+, K+ and high blood pressure. This result could explain a part of phenotype of the patient with CAH.