Abstract: Buckling of carbon fiber reinforced polymer cylinder with circumferential defect was investigated in the current study. Carbon fiber reinforced polymer cylinders with circumferential defect were designed and fabricated. The buckling and failure mechanism of carbon fiber reinforced polymer cylinder with circumferential defect were experimentally and numerically analyzed, and nominally identical cylinder without defect was designed, fabricated, hydrostatically experimented. The results indicate that circumferential concave defects could weaken the sensitivity to defects. Subsequently, a mathematical relationship between defect length ratio and wall thickness ratio for externally pressurized carbon fiber reinforced polymer cylinder with circumferential defect was proposed. An analytical formula for collapse load of such cylinder was derived. Numerical analysis and experimental verification were conducted. The results indicate that the maximum difference between numerical and theoretical results for load ratio of defect to nominal cylinder is 9.7%. The modified theoretical, numerical and experimental data for samples agree favorably. The difference between four kinds of theoretical and numerical results are 1.4%, 4.1%, 1.3% and 9.3%, respectively. The theoretical evaluation for buckling of carbon fiber reinforced polymer cylinder with circumferential defect, is of practical significance.