Introduction: The aim of this study was to evaluate the effect of progressive root canal enlargement on root surface strain during obturation of oval root canals, with either cold lateral or warm vertical compaction techniques. Methods: Sixty-four anatomically comparable human mandibular incisors with oval root canals were classified into four equal groups according to the extent of root canal enlargement. Root canals were cleaned, shaped and enlarged with the ProTaper Universal system (Dentsply Maillefer) with finishing files number 1, 2, 3 and 4, respectively. Each group was further subdivided equally according to the obturation technique: warm vertical compaction (WVC) or cold lateral compaction (LC). All specimens were scanned before and after instrumentation using a multi-detector computerised tomography (CT) scanner to assess root canal volume change. The root surface strain was measured using an electrical strain gauge during obturation. Data for the change in root canal volume was subjected to one-way analysis of variance between groups (ANOVA), while root surface strain data was subjected to two-way ANOVA. Pearson's correlation coefficient was also calculated. Results: There was a significant effect for root canal enlargement and obturation technique, as well as the interaction between them on root surface strain. Significantly higher strain values were recorded during WVC than during LC. Conclusions: Progressive root canal enlargement resulted in higher root surface strain during obturation of oval root canals. Keywords: obturation technique, root canal enlargement, root strain