Light-cured composites and restorative materials are widely used in dentistry. The success of restoration largely depends on the effectiveness of the curing light, including the duration of irradiation and wavelength. This study was conducted to evaluate the effectiveness of curing light devices routinely used in dental offices of Karaj in 2016. Methods: In this descriptive-analytical study, 120 QTH and LED curing light devices used in Karaj dental offices were randomly evaluated. A standard questionnaire was used to collect the variables of the device type, age, and brand, number of repairs, reasons for repair, time of last repair, number of bulb replacements, time of last bulb replacement, device being fitted with a measure severity, duration of irradiation for each composite layer, frequency of using the equipment in a day, sterilization method, light intensity, and number of office days in a week. Data were analyzed with SPSS using Spearman correlation coefficient, independent t test, and chi-square at a level of 0.05%. Results: The results showed that the light intensity of all devices was above 200 mw/cm 2. The light intensity of 11.1% of QTH devices was unfavorable while the light intensity of all LED devices was favorable. The results showed a significant correlation between brand and light intensity, between the number of bulb replacement and device type, between the number of bulb replacements and light intensity, between the number of bulb replacements and clinical age of the device, and between the number of repairs and the device type (P≤0.05). There was no significant relationship between the device type and repair reason. The results also showed an indirect relationship between the clinical age of the device and light intensity. A significant difference was found in light intensity between QTH and LED devices. Conclusion: The light intensity of 11.1% of QTH devices was unfavorable, and the main reason for repair was tip breakage. Light intensity decreased with an increase in the clinical age of the device; therefore, regular quality control and timely bulb replacement are of great importance.