Abstract: Aim To study the difference in mechanical concepts, various accurate expressions and diagnostic technology of viscoelastic and elastic Poisson's ratios. Methods With the aid of correspondence principle of elasticity and viscoelasticity and the integral transform relations the accurate expressions of viscoelastic Poisson's ratio were derived based on some expressions of elastic Poisson's ratio. Results and Conclusion Four accurate expressions of viscoelastic Poisson's ratio were obtained. Some questions of formulation and diagnosis for viscoelastic and elastic Poisson's ratio were analyzed.