The relative hardness (RH) of five composite materials was determined through polymerization via LED lights. Disk specimens were prepared by using composites composed of Artiste, an experimental glass fiber composite, Filtek Supreme, Z100, and LuxaCore. Specimens were polymerized for 10 and 20 seconds from the top surface only with two lights, Smartlite-IQ2 and DEMI LED, with light maintained 8 mm from the surface through a coronal section of the molar tooth. Knoop hardness numbers were determined for the top and bottom surfaces both immediately and 24 hours later. Ten RH values per group were calculated and data were statistically analyzed. Generally, RH increased with increasing polymerization time. The DEMI LED light resulted in RH values that were significantly higher than those obtained with Smartlite IQ2. Z100 had RH values of > 80% under most testing conditions and had the highest hardness values of all composites tested. Therefore, it is considered to be the most appropriate for core buildups.