Abstract: A new kind of portable metal debris monitoring instrument(MDMI) was developed on the base of improving the old direct reading ferrography. The theory of monitoring and every part of the instrument were introduced in the paper. Both submagnetic debris and magnetic debris were tested in the precipitator with MDMI. The results showed that these two kinds of debrises can be monitored by the monitoring instrument. A new quantitative index used for submagnetic debris was put forward, and the parameter of total wear volume was also redefined.