Background and Aim : Inadequate polymerization is one of the main causes of  composite restoration failure .the aim of this in vitro study was to evaluate the degree of conversion  of bulk fill composites  through using FTIR.

Materials and Methods: in this experimental study ,.10 disk shape specimen  from each  material contain X-tra base (Voco, Germany ) and X-tra fil (Voco, Germany ) composites were made in dimension of  2 mm diameter and  4 mm height . The curing procedure was done by LED Turbo (Apoza, Japan ) with intensity of 800mw/cm ² for 20 seconds .Then 1mm from the top and 1mm from the base  of composite specimen cut with diamond disk and degree of polymerization  were evaluated with Fourier transform infrared spectrophotometer (FTIR).The values obtained were analysis by one way  ANOVA.

Results: Although differences in degree of polymerization of X-tra base and X-tra fil in 1mm of top and bottom of composites were significant( p˂0/001) but the mean percentage value of polymerization  on top  and bottom surfaces for both composites were  higher than acceptable range( DC =55%)     

Conclusion: Based on recent results of this study, polymerization degree of all composites  clinically  were acceptable. Although  degree of polymerization of X-tra fil was lower than X-tra base.