- Dendrochronological data provides insight in the tree development, and used in conjunction with forest inventory, can supply inputs for growth and yield modeling. However, accurate and precise measurements of a large number of incremental cores require significant resources. One possible source of valuable tree ring data are national forest inventories (NFI), which process a large amount of data annually. A NFI that places a significant effort on dendrochronological data is the Romanian NFI, which, to date, collected, measured, and cross-dated more than 50,000 incremental cores. This amount of incremental cores makes the Romania the holder of the largest dendrochronological NFI library. The present article presents the method of processing the incremental cores and makes available to the interested entities the tree ring data for the three main species from Romania, namely Norway spruce, European beech, and sessile oak. To ensure utility, the dataset is limited to pure (i.e., one species) even-aged stands (i.e., age difference ≤ 5 years), that were un-thinned (i.e., no active forest management). The data is stored as an ASCII file, and contains six fields: ecoregion, species, year, age, average ring width, and variance . To ensure privacy of information, the average ring width was estimated over a 5 years interval, for which the variance is also computed.