Purpose <p>Despite the aesthetic advantages of composite resins, polymerization shrinkage leads to microgaps, microleakage, and restoration failures. Various light-curing devices and techniques have been developed to reduce polymerization shrinkage; however, microleakage has not been completely prevented. High light intensity can shorten treatment time but may also increase polymerization stresses, potentially leading to increased microleakage. This study aimed to investigate the effects of different light intensities and application directions on microleakage.</p> Methods <p>In this study, 48 caries-free incisor teeth were used. Black V cavities were prepared for the teeth. Teeth were divided into three groups according to the direction of light application (Labial, Incisal, Lingual) and two subgroups according to the different light device types[Woodpecker i LED(2500 mW/cm²), 3&#xa0;M ESPE Elipar (1200 mW/cm²) ]. Composite resins were polymerized from different directions and for different durations. All samples were analyzed for microleakage using basic fuchsin dye and a stereomicroscope. Data were evaluated using the Kruskal-Wallis test in SPSS 20.0 software.</p> Results <p>The type of light-curing device did not result in a statistically significant difference in microleakage (<i>p</i> = 0.274) Microleakage at the gingival margin was significantly higher than that at the incisal margin (<i>p</i> = 0.000) A difference was observed between the groups based on the light application direction, and a statistically significant difference was identified between the incisal and lingual directions (<i>p</i> = 0.002).</p> Conclusion <p>Polymerization shrinkage is a multifactorial problem. High light intensity reduces working time but increases polymerization shrinkage. The direction of light application was found to affect microleakage. For clinical success, light intensity, application direction, and material selection should be considered together.</p>