In order to explore the histological features of yak myocardium during development, we used histological and immunochemical methods to observe the histological characteristics of newborn and adult yaks. The results showed: in newborn yaks, the number of myocardium was less than in adult yaks, the shape was thin and short, the intercalated discs were straight or V-shaped, the type Ⅲ collagen fibers dominated, and type Ⅰ collagen fibers were scarce between myocardium. In adult yaks, the number of myocardium was twice as many as in newborns. The shape was thick and long, the intercalated discs were bamboo-like or ladder-like and distributed between adjacent myocardium, the type Ⅰ collagen fibers dominated between myocardium. ANP was only expressed in atria myocardium, with expression quantity greater in the right atrium than in the left atrium, and in newborns more than in the adult yak. Less ANP was expressed in Pukinje fibers and the expression of ANP was negative in the left and right ventricular muscle. The expression of Cx43 in myocardial membrane and cytoplasm in newborns were greater than in the adult yaks, but at the intercalated discs, the adult was greater than the newborn yaks. Cx43 was positive in membranes of Pukinje fibers in newborn and adult yaks. These results indicated that the shape of the myocardium became thicker and longer, the number was increased, ANP expression in atrial muscle was decreased, and Cx43 expression in intercalated discs was more remarkable, the compensatory capacity of adaptation cycle change was progressively reduced; and the intercellular junction was strengthened, that could increase the speed of the impulse conduction among myocardium in the adult yaks compared with newborn yaks,