Photodissociation dynamics of HN3 at 157.6 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been measured. From these distributions, three H-atom channels are observed. The vibrational structure in the fast-H channel could be assigned to a progression in the N-3 symmetric stretching mode (v(1)00), together with a progression of the symmetric stretching mode with one quantum of bending motion (v(1)10). The broad translational energy distribution of the slow-H channel is energetically consistent with the cyclic-N-3 formation process or a triple product dissociation channel. Photodissociation of DN3 was also investigated using the same technique. Isotope effect on the product translational energy distribution has been observed, in which the slow H-atom is clearly more pronounced.