Tissue samples from the breast height (1.3 m) of the selected chestnut trees (Castanea sativa Mill.) growing at a site of Monti Cimini in the Lazio region (Italy) were collected from 16 March until 28 October 2009. The samples were taken with the micro-coring method at weekly intervals. Microscopic cross-sections that contained xylem, cambium and phloem were prepared and analyzed with the light microscope Nikon Eclipse 800 (bright field and polarized light), digital camera DS-Fi1 and image analysis system NIS ElementsBR3. The width of the xylem increment of the current annual ring was measured on samples taken at weekly intervals. The phase of cell differentiation was recorded as well. Occurrence of the first vessels was detected between 31 March and 7 April 2009. The synthesis and lignification of their secondary cell walls started between 23 and 29 April 2009. They started around the first vessels and spread afterwards to the surrounding tissue. The most intensive rate of xylem formation was detected from the beginning of June until the beginning of July. Earlywood consisted of 2-3 rows of vessels; its proportion was larger in narrower tree rings. Latewood started to develop in the period between 20 and 29 May and was fully developed by the week from 24 September till 1 October. Tree ring formation lasted for approximately 4 months, considerable differences among the trees were observed. The final average tree ring width was 3648 m (3516 703 m). The results of xylem growth were fitted with Gompertz, Chapman-Richard, Weibull and Korf growth functions. All of them were useful for fitting the data, but the Gompertz function seemed to be the most appropriate. The culmination calculated with this function, optimally agreed with the actual culmination of the increment. The parameters of this function are also very useful for describing the formation of the tree ring.