The perfect hexagon showed by columnar jointed basalts gets great public appeal.However,the convincing explanation remains unresolved.This contribution proposes a new explanation for its origin by stress analysis and rigid calculation.With the cooling of basalts,the potential rupture and micro-deformation generates while the normal stress equal to the tensile strength of basalts.Because the Poisson effect,the distribution of stress at the micro-deformation changes and new potential rupture appears.With the growing of tangential stress,the condition of the shear fracture happens when it equals to the shear strength of basalts,which shows the perfect hexagon structure of columnar jointed basalts.According to previous relevant experimental data,we get the internal friction angle,the cohesive force,the tensile strength and Poisson’s ratio of basalts at the corresponding temperature.By Stress Mohr circle method,the numerical calculation shows that cleavages appear at about 800 ℃ and the hexagon columnar begin to generate with ~119.1°angle among the cleavages.By the further analysis,it shows that the angle and process may depend on the cohesive force and the percentage composition of quartz mineral.