Purpose : To evaluate the effect of enamel prism orientation on microtensile bond strengths (MTBS) to enamel using two current bonding systems. Materials and Methods: The crown segments of extracted human third molars were cut mesio-distally to obtain buccal and lingual halves. One of the halves was cut horizontally at the middle of the crown to make a flat enamel surface parallel to prism orientation (parallel specimen). The other half was ground at the equator of the crown to produce a flat enamel surface perpendicular to the prism orientation (perpendicular specimen). Each surface was ground with 600 grit silicone carbide paper, treated with Clearfil Liner Bond IISigma (LB) or Single Bond (SB) according to the manufacturers' instructions and resin composite, AP-X or Z100, respectively, was built up onto it. After storage in water at 37 degreesC for 24 hours, all bonded specimens were sectioned and trimmed for measuring MTBS. In MTBS, the parallel specimens were stressed perpendicular to the prism long axis and the perpendicular specimens were stressed parallel to the prism axis. Results: MTBS of specimens stressed perpendicular to the prism long axis were 11.93 +/- 5.56 MPa for LB and 10.97 +/- 1.80 MPa for SB. MTBS of specimens stressed parallel to the prism axis in LB and SB were 19.07 +/- 6.35 MPa and 23.99 +/- 9.20 MPa, respectively. For each prism orientation, no difference in MTBS was found between the bonding systems (P > 0.05). Both systems showed lower MTBS in specimens stressed perpendicular to the prism long axis than in specimens stressed parallel to the prism axis.