Purpose: The purpose was to evaluate temperature increases during dowel space preparations with oval and circular fiber dowel systems. Materials and Methods: This study included 42 single-rooted human mandibular premolars. Roots were scanned with cone-beam computerized tomography (CBCT) to determine the ovoid root canal morphology. Root canals were treated with Ni-Ti rotary instruments and obturated. A second CBCT was taken to determine the thinnest dentin thickness of each root. Roots were randomly divided into two groups (n = 21) according to the fiber dowel system used: group 1, circular fiber dowel system (D.T. Light-Post); group 2, oval fiber dowel system (Ellipson Post). Dowel spaces were prepared using a circular fiber dowel drill and a diamond-coated ultrasonic tip with an oval section under water cooling until 9 mm dowel spaces were obtained. Temperature changes were recorded from the thinnest root surfaces using a FLIR E60 thermal imaging camera. Results: Temperature increases were significantly greater with the circular fiber dowel system than with the oval fiber dowel system (p < 0.05). Conclusions: Although both dowel systems generated high temperature increases on root surfaces, the relatively lower temperature increase associated with the use of oval fiber dowels in ovoid canals makes it preferable to the use of circular fiber dowels.