It is of practical importance to develop the tetra-needle-shaped ZnO whisker (T-ZnOw)-reinforced polymer composites that have isotropic properties. To give a guidance of material design and manufacture, it is necessary to have a theoretical analysis of the fracture mechanisms of this peculiar structure fiber in polymer composites. Based on previous investigations of the TZnOw-reinforced polymer composites, and from the viewpoint of materials mechanics, here we analyzed the distribution of stresses on different points of the tetra-needle-like crystal whiskers in a composite and calculated the total stress at the connection point. The results indicate that the stress on the connection point is proportional to the exerting force and correlates with the dimension, the size, and the location of the whiskers. According to the theoretical derivations, it was found that the stress at the connection point of the T-ZnOw is much larger than that at the others, leading to breakage on that point mainly or wholly, which is in accordance with the experimental observations. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2767-2771, 2011