Phantom-based evaluation of geometric distortions in functional MRI and diffusion tensor imaging (DTI) was investigated. An acrylic water-filled phantom with a grid structure was designed and manufactured to provide accurate geometric information over the volume measured in human brain imaging. The grid structures were well detected in data acquired using a 3-T MRI scanner with echo-planar imaging (EPI) sequences commonly applied in functional MRI and DTI. A method for quantifying distortions in the phantom data was presented and applied for the images. The validity of the phantom for EPI was evaluated by quantitatively comparing the distortions present in and induced by the phantom and a human brain when imaged under identical conditions. The results suggest that the new phantom can reveal geometric distortions easily undermined by standard MRI phantoms. For example, prominent variability in the distortions was found as a function of the orientation of the diffusion-sensitizing gradient. Possible future applications for this type of phantom include quality assurance and calibration of the hardware and software used in EPI-based functional MRI and DTI.