In the present study the effect of insoluble diluents such as microcrystalline cellulose (MCC) and dibasic calcium phosphate (DCP) on the compression characteristics and release profile of sustained release tablets containing Hydroxypropylmethylcellulose (HPMC) matrices was investigated. The effect of diluents on the compression characteristics was studied using Heckel and Kawakita equations. The effect of compression forces on the release profile was also investigated. Diclofenac Sodium (DS) was used as a model drug. Tablets were prepared using wet granulation method. It was found that there is a decrease in the drug release as the concentration of these insoluble diluents is increased. From the Heckel and Kawakita analysis it was concluded that the compressed granules of formulations containing microcrystalline cellulose showed higher plastic deformation, densification and granule fragmentation as compared to DCP. Also a relationship was evaluated between the compression force and the release profile i.e. an increase in compression force causes decrease in the release rate of the drug from the formulation irrespective of change in the diluent. (C) 2011 Elsevier B.V. All rights reserved.