Objective: The purpose of this in vitro study was to evaluate heat generation with the use of LED curing device externally (on the outer surface of the tooth)& internally (inside the pulp chamber), in compares with plasma arc & conventional curing device Materials & Methods: On the buccal and palatal surface of fifteen upper extracted premolar teeth a standardized class V cavity was prepared, the teeth were randomly divided into three groups according to the cure mode, group I cured with DentSuply (Conventional cure unit QTH), group II cured with Radii (LED cure unit) and group III cured with Flipe .( Plasma arc cure unit) . Each group consist of 5teeth (10 cavities), and filled with (micro-hybrid resin composite), before light curing of the composite resin a thermocouple was inserted into the pulp chamber .A second thermocouple was attached on the surface of the tooth; the other end of the thermocouple was connected with anelectrical thermometer. Two reading of temperature increasing were measured. Then the difference between the initial and final temperature was recordedexternally and internally.Results: Statistical analysis of the results revealed that the LED light curing unit had the least increase in the temperature with a significant difference (P<0.001)when it was compared with plasma arc curing device externally & internally. In spite of statistically a significant difference between LED & conventional curingdevice externally (P<0.001) and the lower temperature mean was found with LED internally, no significant difference between LED & conventional curingdevice internally.Conclusion :LED light curing unit had the least increase in the temperature at the outer surface of the tooth and inside the pulp chamber.