The objective of the present study was to evaluate the role of myofiber characteristics and the thickness of 2 major muscle membranes, perimysium and endomysium, in determining the breast meat tenderness of chickens. Birds from 2 breeds (White Leghorn and a line of broiler) were chosen. Chicks were sexed and wing-banded at hatch and were grown in separate cages in a single house. Sixty broilers and 60 White Leghorns were harvested at 6 wk of age, respectively, whereas another 60 White Leghorns were slaughtered at 18 wk of age. An equal number of males and females was maintained for each group. Body weight, breast muscle weight, pH, drip loss, cooking loss, Warner-Bratzler shear force value (SFV), total energy of shear force, fiber diameter, sarcomere length, myofiber density, and the thickness of endomysium and perimysium of the breast were determined for each bird. At 6 wk of age, histological examination indicated that the size of myofiber and thickness of endomysium and perimysium of broilers were larger than that those of White Leghorns (P < 0.01), whereas the SFV, drip loss, and cooking loss of broilers were smaller (P < 0.01). A comparison between the White Leghorns at 18 wk and the broilers at 6 wk, which were at similar BW but different ages, showed that the breast muscle weight of broilers was larger (P < 0.01) than that of White Leghorns. For breast muscle, the endomysium of broilers at 6 wk was thicker than that of White Leghorns at 18 wk (P < 0.01), whereas the perimysium was thinner (P < 0.01). The SFV, drip loss, and the cooking loss of broilers were smaller than those of White Leghorns at similar BW (P < 0.01). Meat tenderness was negatively correlated with myofiber density (-0.27) and the thickness of endomysium (-0.29) and positively correlated with the thickness of perimysium (0.20). It is suggested that muscle membrane should be considered in evaluating meat tenderness of the chicken.