Backgroung: Multiple beams are generally used with an increased possibility that the beam axis intersects the treatment table. Treatment tabletops are commonly made of carbon fiber due to its high mechanical strength and rigidity, low specific density, extremely light and low radiation beam attenuation properties. Purpose of this paper is investigated the dose changes in the buildup region and beam attenuation by a carbon fiber tabletop for high energy 6- and 18-MV photon beams. Materials and Methods: Measurements were performed for 10 cm x 10 cm and 20 cm x 20 cm field sizes. The surface dose and percentage depth doses (%DD) were measured by a Markus parallel plate chamber at a source-surface distance (SSD) of 100 cm for 6 MV and 18 MV photon beams. Attenuation measurements were made at the solid-water phantom for gantry angles of 0 degrees and 180 degrees rotation of the beam. Results: A carbon fiber tabletop increases the surface dose from 12.87% to 86.65% for 10 cm x 10 cm and from 8.72% to 71.16% for 20 cm x 20 cm field at 6 and 18 MV, respectively. The surface dose with the carbon fiber tabletop in an open field (0 degrees) increases with field size. Conclusion: The carbon fiber tabletop causes a substantially increased surface dose, and also significantly decreases the skin-sparing effect, which is clinically important. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation.