A single center, prospective, randomized, single blinded, controlled study comparing the effects and safety of "sitting pelvic tilt exercise" in relieving back pain during the third trimester in primigravida was carried out. The samples were composed of 67 primigravidas who attended the prenatal clinic at King Chulalongkorn Memorial Hospital. All subjects were selected by the random sampling technique and allocated into two groups for the experimental group and the control group; for 32 and 35 pregnants, respectively. The experimental group received the pelvic tilt exercise program for 8 weeks during the third trimester. Pain intensity was measured by visual analogue scale (VAS) at day 0 and day 56 in both groups. The result of the study revealed 1) The mean VAS of back pain in the experimental group was significantly lower at day 56 than at day 0 and lower than the control group at day 56 (p< 0.05) by unpaired t-test 2) There was no incidence of preterm labor, low birth weight or neonatal complication in the experimental group. In conclusion, the "sitting pelvic tilt exercise" during the third trimester in primigravidas could decrease back pain intensity without incidence of preterm labor, low birth weight or neonatal complication.