Purpose: The aim of this study was to compare postoperative sensitivity following placement of posterior composite restorations using the fast- or step-curing modes of an LED curing light. Materials and Methods: Thirty patients participated, with each having two homologous contralateral posterior teeth with Class II carious lesions. One restoration was cured using the fast-curing mode of the LED curing light (Mini LED), and the contralateral restoration cured using the step mode of the same curing light. The patients were contacted after 2 and 7 days postoperatively and asked about the presence or absence of sensitivity on a scale from 0 to 3. 0: no sensitivity, 1: slight sensitivity, 2: moderate sensitivity, and 3: severe sensitivity. If the patient experienced sensitivity at 7 days postoperatively, he/she was contacted again after 30 and 90 days. Results: There was a statistically significant difference in postoperative sensitivity between the two curing modes at days 2 and 7 postoperatively (p 0.05) but not at days 30 and 90 (p > 0.05). The intensity of sensitivity was also different between the two curing modes at days 2 and 7 postoperatively (p 0.05) but not at days 30 and 90 (p > 0.05). Conclusion: The step mode of the LED curing light reduced the incidence and severity of postoperative sensitivity following placement of posterior composite restorations compared to the fast mode of the same curing light. Keywords: clinical trial, resin composites, LED curing lights