With increased use of halftone dot overprints for offset color printing, it is important to study the density and ink trap of the overprints. In this research the equation to predict the density and the method to evaluate ideally the fractional ink trap are preposed. And also the halftone dot overprinting experiments of Magenta over Cyan or Cyan over Magenta under wet-on-wet or wet-on-dry overprinting using 2-color offset press show the above proposals are reasonable, and show the effect of overprinting sequence on the density and ink trap.