Abstract: External plastic structure of a treadmill includes a base seat and a running platform. The base seat has two columns respectively upward extending from two sides of the base seat. A panel plastic member is disposed between the two columns. A front section of the running platform is mounted between the columns. A running belt is circularly rotatably disposed in the running platform. A step board plastic member is disposed on two sides of the running belt of the running platform. The panel plastic member and the step board plastic member are made of plastic material by blowing molding, whereby internal buffering spaces are formed therein.