Narrow-leaved ash (Fraxinus angustifolia) is one of the important broadleaved tree species, and it is becoming more important in European forestry because of its valuable wood and fast growing ability. Despite its wide natural range and high economic value, there is little or very limited information about the effects of thinning on the growth and development of ash stands, especially in plantations. In this study, 2 thinning experiments were carried out to determine the effects of thinning intensity on the growth of diameter, height, basal area, and volume in narrow-leaved ash plantations over a 6-year period in Adapazarı, Turkey. In the stands prior to thinning, mean diameter and stem number were about 31 cm and 416 trees ha 1 in the first experiment (at 36 years with 3 × 2 m initial spacing), respectively. The values were 24 cm and 544 trees ha 1 in the second experiment (at 22 years with 3.7 × 3.7 m initial spacing), respectively. Randomized block design with 3 replications was used in both experiments. The thinning treatments were as follows: removal of the basal area at 0% (control), 22% (moderate), and 39% (heavy) in the first experiment, and 0% (control), 19% (moderate), and 28% (heavy) in the second experiment. The 6-year results showed that thinning increased the diameter increment significantly, and the increase in diameter increment was positively correlated with the thinning intensity in both experiments. However, thinning intensity did not significantly affect increments of height, basal area, and volume. Moreover, increments of diameter, height, basal area, and volume were higher in the second experiment than in the first experiment.