Abstract: An internally-sealed one-piece external thread ball valve includes a valve seat gland and a left sealing gasket. The valve seat gland and the left sealing gasket are installed into a valve body. The contact part of the valve body and the valve seat gland is an internal thread. The contact part of the valve body and the left sealing gasket is a smooth inner circular surface. The left sealing gasket is in an inverted trapezoidal shape with a thin inner part and a thick outer part. The left sealing gasket expand outwards under an action of pressure formed by tightening a left nut during valve assembly. The outer circular surface of the left sealing gasket and a smooth inner circular surface of the valve body form sealing.