Objectives: Composite resin materials shrink during polymerization regardless of curing unit or polymerization program used. However, different polymerization programs lead to different shrinkage percentages. The aim of this study was to evaluate linear polymerization shrinkage for five composite materials polymerized with curing modes of two LED curing units. Method and Materials: Bluephase C5 (500 mW/cm2, 20 seconds) and Bluephase C8 (high-power mode—800 mW/cm2, 20 seconds; and soft-start mode—650 mW/cm2 for the first 5 seconds, and 800 mW/cm2 for the next 15 seconds) (both curing units by Vivadent) were used for composite resin photopolymerization. The materials used were two fine-particle hybrid composite resins (Tetric Ceram and Artemis) and a nanohybrid composite (Tetric EvoCeram) of different shades (all by Vivadent). Polymerization shrinkage was recorded during each second of the polymerization process using laser interferometry. The results were analyzed by ANOVA, and Tukey post hoc test was used for finding significance levels. Results: The highest shrinkage values were obtained in the case of composite polymerization using the high polymerization mode of Bluephase C8 curing unit (0.61% for Tetric EvoCeram A1 and A3 and 0.69% for Artemis A2), while the lowest results were observed with the soft-start mode of Bluephase C8 LED (0.48% for Artemis A2, 0.41% for Tetric Ceram A3, and 0.35% for Tetric EvoCeram A3.5) and Bluephase C5 curing unit (0.40% for Tetric Ceram A1 and 0.44% for Tetric EvoCeram A3.5). Artemis A2 showed the greatest overall sample dimension change, regardless of the light-curing source used. The initial expansion was recorded for all composite resins and curing modes during the first 4 to 9 seconds of illumination. Conclusion: Negative consequences of polymerization shrinkage and temperature rise can be diminished by applying a polymerization program with lower initial light intensity, which prolongs the pregel phase of the polymerization process. (Quintessence Int 2010;41:417–425)