Amalgam and composite resin materials are considered the main durable direct restorative materials. Diametral tensile strength (DTS) of any restorative material is an essential test that could reflect the tensile behavior of the restorative material during function in the oral cavity. This strength could be affected by the use of bleaching agents used nowadays for the purpose of tooth whitening and consequently might interfere with their longevity. The aim of this study was to evaluate the effect of in-home bleaching material on the DTS of an amalgam and two composite resins. Two types of light activated composites, TG fine glass (Technical & General Ltd, Germany); Cavex (Quadrant Universal LC, CE 0197; Germany)) and one type of amalgam; World-Cap (Ivoclar vivadent FL-9494 schaan/Liechtenstein, Sweden) were used in this study. Forty resin composite specimens and 20 amalgam specimens were prepared using a circular nickel-chromium split mold with 6 mm in inner diameter and 3 mm in height. The twenty samples of each restorative material being selected were divided into 2 groups: ten tested before treatment with home bleaching tooth whitening system and the other ten tested after treatment with home bleaching system (WHITE smile HOME BLEACHING, 35% carbamide peroxide, Germany) for 8 hours: (4×2 hours). Specimens were placed into a dark bottle containing distilled water at 37°C for 7 days before testing procedure. DTS testing was performed in a Universal Testing Machine at a crosshead speed of 0.5 mm/min. The data was calculated in MPa and data were analyzed by t-test at 0.05 level of significance. World-Cap amalgam restorative material being tested, exhibited statistically insignificant differences (P≥0.05) in diametral tensile strength values between the groups before and after bleaching while TG and Cavex restorative composites, exhibited significantly lower DTS values after bleaching compared to the values before bleaching. DTS values of the two composite restorative materials being tested is significantly influenced by the bleaching agent.