A method to determine the non-linear viscoelastic constitutive constants from indentation force displacement data corresponding to different indentation speeds has been developed. The method consists of two parts. In the first part, the force displacement data is expressed as two functions which represent the strain and the time-dependent responses, respectively. From these functions, the time-dependent constants and the instantaneous force displacement response are obtained. In the second part, the strain-dependent variables are determined from the instantaneous force - displacement response through an inverse analysis based on the Levenberg - Marquardt method. The method was verified by numerical experiments using the properties of cheese as examples.