Cast-in-place piles in the field are likely to have unintended defects during construction, such as soft bottom, segregation of concrete, and contraction or enlargement of the cross section. The effect of defects in cast-in-place piles on the pile's resistant behavior was evaluated. Four piles with artificial defects of soft bottom, concrete segregation, and contractions of the cross section by 10% and 20%, respectively, as well as one sound pile, were constructed simultaneously. Static load tests, load transfer measurements, and integrity tests were performed for the piles. In addition, three-dimensional numerical analysis was conducted to analyze the effect of the contraction defect on the pile's resistant behavior. For the pile with the defect of concrete segregation, integrity tests and load transfer measurements showed that the pile strength was reduced by more than 50% in the section of the weak zone having the defects. For the piles that contained the defect of asymmetric contraction of the cross section, the measured strain in the defective section was lower than that in the section without the defect, and three-dimensional numerical analysis showed that the defect increased the axial stress in the section of the defect. Crosshole sonic loggings were conducted before and after static load tests. The difference in the loggings between the prestatic and poststatic load tests indicated the growth of defects as the piles were loaded.