Naturally fractured reservoirs (NFR) have been receiving more attention than ever since the beginning of the last decade due to various reasons. The current understanding is not sufficient to achieve a favorable recovery factor, due to the lack of effective characterization and the complex dynamic nature of the fractured system. The fact that NFR are found in many countries around the globe in almost every lithology is another justification for more interest. The majority of the fractured reservoirs around the world are developed. Therefore, before proceeding into secondary or possibly tertiary recovery processes a thorough understanding must be accomplished to avoid undesirable results. The fracture characterization is the first building block in any NFR study. This study utilized well test data to detect fractures between two communicating reservoirs. The study was inspired by a real field example in which two reservoirs, separated by a thick non-reservoir formation, are in hydraulic communication with each other. A refined simulation model was constructed to predict the type of response that would be observed in such communicating reservoirs during a well test. This paper presents the result of this study, which showed a unique shape on the derivative due to the communication through fractures. The investigation was taken further to study the impact of the first and the third layer permeabilities on the observed shape of the derivative. The impact of fracture length, fracture permeability, fracture distance from the well, and the existence of multiple fractures were also investigated.