The key imaging parameter of number of diffusion gradient directions (NDGDs) in the protocol of diffusion tensor imaging (DTI) is significant in the acquisition of raw data. Up to date, the selection of NDGD in DTI is still in controversy for the optimized NDGD to observe 3D deterministic and probabilistic fiber trackings (FTs) of DTI in clinical evaluations. In this study, we evaluate the NDGDs of DTI on 3D deterministic and probabilistic fiber trackings (FTs). One volunteer was scanned by DTI protocols with five NDGDs of 6, 11, 15, 21, and 31 orientations, respectively. In 3D, the two deterministic and probabilistic FTs were qualitatively observed by delineating the region of interests (ROIs) of single-voxel and multiple-voxels. The results showed that the major fiber branches could be discovered with NDGD of six directions and the tracking performance could be easily affected by image noise; moreover, NDGD above 11 orientations could provide detailed fiber tract configurations for showing minor fiber tracts with longer scanning time.