Pressure-Volume-Temperature (P-V-T) properties of two amorphous polymers (ABS and PS) were measured by an on-line testing measurement in the temperature ranges from 20 to 150 °C and pressures from 20 to 120 MPa. The on-line testing measurement is based on an injection molding machine (IMM), and it can be used to get P-V-T data of polymers directly with a special testing mold under normal processing conditions. The experimental P-V-T results of amorphous polymers under industrial processing conditions were presented. A new modified two-domain Spencer-Gilmore equation of state (EOS) was developed. The P-V-T properties were correlated by the modified Spencer-Gilmore EOS and a modified two-domain Tait EOS, showing a good agreement with measurements. The effects of sample forms were investigated. A sample with large length diameter ratio is the best one to be measured.