Purpose To investigate the effect of head tilt on keratometric measurement using the IOLMaster. Methods Twenty-seven right eyes of 27 volunteers were examined using a manual keratometer (MK), automated keratometer (AK), and an IOLMaster. MK and AK measurements were performed in the upright head position, whereas IOLMaster measurements were performed in five different head positions (upright, 51 or 151 clockwise head tilt, and 51 or 151 counter-clockwise head tilt). The repeatability of IOLMaster was compared with MK and AK. The keratometric measurements (mean keratometric power, magnitude of astigmatism, and steep meridian) in different head positions were compared. Results The IOLMaster showed good repeatability of keratometric measurement comparable to MK or AK. 151 and 51 clockwise head tilt resulted in 12.09 ± 9.51 (mean ± SD) (Po0.001 vs upright) and 5.51 ± 5.97 (mean ± SD) degrees (Po0.001) of clockwise rotation of steep meridian, respectively. 151 and 51 counter-clockwise head tilt resulted in 12.49±7.07 (mean±SD) (Po0.001) and 6.08±5.09 (mean±SD) degrees (Po0.001) of counter-clockwise rotation of steep meridian, respectively. Conclusions The patient's head tilt (51 or 151, clockwise or counter-clockwise) significantly changed the steep meridian of astigmatism measured using IOLMaster and the steep meridian generally shifted to the direction of head tilt. The importance of maintaining the upright head posture during IOLMaster measurement is demonstrated.