Objective To derive the circumferential stress on elastic tube in a biologic body using mathematics and mechanics analysis. Methods To calculate the formula of circumferential stress (σ θ) on an elastic tube. Results The σ θ on elastic tube is approximately equate to ΔP · r o/D(ΔP,r o and D are the pressure,the mean radius of the tube wall and the thickness of the wall,respectively),with the absolute error δσ θ=ΔP/2,and relative error δσ θ/σ θ = D/2r o.The semi-exactitude formula of the circumferential stress was ΔPrD-P 1+P 22,with the absolute error δσ θ=ΔP2-(P 1+P 2)D2r 0,and relative error δσ θ/σ θ=1σ θΔP2-D(P 1+P 2)2r 0.It was found that the highest circumferential stress exists in small artery of human. Conclusion The approximate formula of circumferential stress and the absolute error is accepted,which is easy for the application.The small artery is trend to be split due to the highest circumferential stress present.