Objective: To evaluate the effect of composite preheating and light-curing duration on hardness and postgel shrinkage. Method and Materials: A hybrid composite (Filtek Z250, 3M ESPE) and a nanofilled composite (Filtek Supreme Plus, 3M ESPE) were preheated in a Calset (AdDent) heating device at its highest setting (68°C) before being light cured for 5, 10, 20, or 40 seconds. Vickers hardness of the top (0 mm) and bottom (2 mm) surface of the samples was measured 5 minutes after polymerization. The development of postgel shrinkage during polymerization was measured with a strain gauge. The results were compared to those of samples cured at room temperature. The sample size was five for each composite/temperature/light-curing duration combination. The results were statistically analyzed with two-way ANOVA and least squares means. Results: Preheating composites before light curing did not affect the bottom hardness or postgel shrinkage (P > .05) but significantly increased the top hardness of the Supreme Plus composite at all light-curing durations (P .05). Shorter light-curing durations than recommended by the manufacturer significantly reduced top and bottom hardness of both composites, regardless of preheating (P .05). Conclusion: Under the experimental conditions in this study, preheating of the composites only slightly increased hardness values and did not negatively affect postgel shrinkage. Reducing the light-curing durations, however, significantly reduced the hardness at both the top (0 mm) and bottom (2 mm) surfaces. Clinicians should ensure sufficient light-curing duration to obtain good mechanical properties rather than depending on composite preheating. Keywords: Calset, composite, hardness, light-curing duration, preheating, shrinkage