In this paper fiber/matrix interface debond growth in unidirectional composites subjected to mechanical tensile loading is analyzed using fracture mechanics principles of energy release rate (ERR). The objective of the present study is to analyze the effect of neighboring fibers on the ERR. 5-cylinder axisymmetric FEM models with adjustable inter-fiber distance were used for ERR calculations. The results show that the ERR slightly increases with the inter-fiber distance in the case of long debonds. For short debonds, however, because the stress-state is more complex, it was found that the debond propagates in a mixed Mode I and Mode II and contribution of each mode to the ERR depends on the actual debond length. It was found that for very small debond lengths ERR significantly increases with the inter-fiber distance.