Deformation and relaxation response of biomedical polymers on application and removal of stress, respectively, are critically important in the clinical durability of prosthetic appliances using these materials. The material responses are both short-range and long-range. Although short-range responses can be studied in short-time experiments, the longterm responses are critically more important to assess risks in longterm clinical applications. The objective of this investigation was to use WLF methods of time-temperature superposition to study longterm creep and stress relaxation in dental composites. While composite matrix and filler components contribute to creep and stress relaxation, the major differences in dental composites are due to filler/resin volume ratio and filler particle size and distribution effects. In this study, three distinctly different composite materials were evaluated for longterm creep and stress relaxation effects. The composites included a microfilled composite, a mini-particle hybrid composite and a small-particle hybrid composite. Creep and stress relaxation effects were monitored for 10-minute durations at 5degreesC intervals between 32 and 117 degreesC in a TA Instruments DMA model 2980. The validity of WLF equation to creep compliance and stress relaxation data was analyzed using least square method. The results show excellent fit of creep and stress relaxation data shift as per WLF model. Extended master curves for creep compliance and stress relaxation were computed and analyzed. The longterm creep and stress relaxation effects were two to five times higher than the corresponding short-time values. Creep and relaxation effects of different composites also showed wide differences, indicating influence of filler and resin contributions to the overall behavior.