Abstract: By discussing on star frame of reference and center-of-mass frame of reference,the conditions that star frame of reference and center-of-mass frame of reference are inertial frames,are indicated.Two doubts about the system of Newton＇s mechanics are resolved.

Key words: inertial frame, local inertial frame, star frame of reference, center-of-mass frame of rererence, gravitational, inertial force, tidal force