Background: The aim of this in vitro study was to evaluate and compare the effect of preheating microleakage among three different filler size composites which include Filtek^tm Z250 micro hybrid, Z250^xt Nano hybrid and nanocomposite Z350^xt.  in Class II cavity preparation .

Materials and methods: sixty maxillary first premolars were prepared with class II cavities. Samples were divided into three groups according to material used    group A (FiltekZ250 micro hybrid). Group B(Z250^xt Nano hybrid). Group C (nanocomposite Z350^xt)and each group divided into two subgroups of ten teeth according to temperature of  composite: Group A1 Teeth are restored by composite at  room temperature(24±1ËšC), A2 Teeth are restored by same composite at preheated temperature(54±1ËšC). After 24 hrs. immersion in 2% in methylene blue, samples were sectioned and micro leakage was estimated.

Results: The greater scores of micro leakage mentioned in Group A( Z250 micro hybrid) in cervical margin while in Group B( Z250^xt Nano hybrid) and  Group C (Z350^xt nanocomposite) have least score of micro leakage especially in occlusal margin.

Conclusions: The scores of micro leakage for all groups varied with different material, margin and temperature. Generally, preheating decreased micro leakage  in all groups of composites ,but Preheating decreased micro leakage effectively in Z250 micro hybrid more than Z250^xt Nano hybrid and Z350^xt nanocomposite micro leakage .