Since the creation of the first reserve in 1997, a conservation plan allowed the development of a network of Forest Nature Reserves in Tanzania over the years. Nowadays the network comprises 20 declared reserves (and two reserves in the process of being announced), each characterized by various management plans, ecoregions, threat levels, budgets, and sizes. Despite improvements in the number of reserves declared, management efficacy and participation of local communities, some threats are still impacting the biodiversity level of the reserves, these include a lack of funding for management, deficiencies in terms of equipment and personnel, and lack of benefits to the local communities. Thus, our objective was to use the data collected previously, to evaluate and improve this network of Forest Nature Reserves. To accomplish this, first, the parameters significantly related to the biodiversity value of the reserves were identified, and second, the taxonomic groups (among amphibians, reptiles, birds, and mammals) poorly covered by the reserves and the location of possible gap(s) in the network were detected. To complete the first objective, we achieved some descriptive analysis, using Microsoft Excel and R, and for the second objective, we used the spatial software QGIS, to perform a gap analysis and build species richness maps. Our results demonstrated that ecoregions were the most important factor being significantly related to the biodiversity index of the reserves, with the Eastern Arc Mountains being the most valuable forest in terms of its biodiversity amount. In addition, the gap analysis revealed that the proportion of Tanzanian endemic gap amphibian and reptile species was higher than in other taxa in the network, showing a conservation bias. Secondly, the species richness maps identified gap areas, where other types of small, protected areas were found, which could be upgraded in the future to capture additional endemic/threatened species. To conclude, despite an improvement in the development of the protected areas over time in Tanzania, there are still some weaknesses in the network that should be considered in the future.