This study was to determine the quality of curing of second generation light emitting diode (LED-II) compared to a first generation LED (LED-I) and conventional halogen (quartz tungsten halogen) light curing unity by measuring depth of cure, degree of polymerisation and surface hardness. Filtek Z-250 hybrid composite resin (3M-ESPE, Germany), with shade A2, was used in this study. One quartz tungsten halogen/light curing unit (LCU) (Hilux, Benlioglu Dental, Turkey), one LED-I-LCU unit (Elipar Freelight 3M-ESPE, Germany) and one LED-II-LCU (Elipar Freelight, 3M-ESPE, Germany) were used for polymerisation of the composite resin. The depth of cure was obtained with scrape test, degree of cure was measured with Fourier transform infrared spectrometer (Spectrum-One, Perkin-Elmer, USA) and surface hardness was calculated with surface hardness tester (Micromet-2001, Buehler, Dusseldorf, Germany). The data were analysed by Kruskal Wallis and Mann Whitney-U. The polymerisation depth of LED-I-LCU was significantly lower than that of the other LCUs (p<0.05). However, no significant differences were obtained among all tested systems in point of degree of cure and surface hardness at 2 mm depth. Within the limitations of this study, although LED-I showed lower depth of cure, all tested systems appear suitable for routine clinical practice for polymerisation of the composite resin.