A rocker assembly of a strapping machine has a rocker having a seat, a positioning panel mounted on the seat and being able to swing, and two teeth panels slidably mounted on the positioning panel. When a fixed end and a tightening end of a wrapping strap are mounted between the rocker assembly and a tensioning wheel, the teeth panels slide and the positioning panel swings to adjust engaging surfaces and engaging forces between the teeth panels and the wrapping strap according to thickness of the wrapping strap. Thus the wrapping strap is able to securely wrap around the goods and is not scratched.