Abstract: Vertical,horizontal and shear components of ground stress,elastic modulus and Poisson’s ratio are five important parameters for plane problems of underground engineering.The present paper makes further investigations about the number of the parameters that are possibly identified by the elastic back-analysis from displacements,using the analytic displacement solution of a single tunnel.It is found that the inversion isn’t unique when all ground stress components and elastic modulus are unknown parameters to be back-analyzed.Besides this case,all the unknown parameters are possibly inverted only that the inversion for tunnel with simple geometry places more limits on the parameters to be identified.