||In the hard disk drive, MR head has been widely used as the reading transducer. As the area1 recording density increases, the transducer dimension must be reduced, and the signal output of the MR head becomes smaller. To maintain the signal level, some improvements to the current MR head have to be done in the industry. However, the improvements are often constrained by electron migration reliability. Two approaches were found to be effective to improve the MR head signal output. One is the reduction of dummy contact resistance of metallic joints of MR head. The other is optimization of the film thickness and working current. However, no much study on the contact resistance was reported in the past. Recently, the observation by TEM confirmed that the contact resistance was caused by the contamination in the interface. To monitor and improve the contact resistance in the head fabrication process, a macroscopic resistance measurement method was developed to reveal the contact quality instead of TEM. This method shows a good agreement with the direct TEM observation. To select the MR thickness and working current, a new idea to assess output in reliability lifetime scale was proposed. A systematic method was applied to predict the reliability lifetime as well. By this method, we demonstrated the method to obtain the optimal selection of the MR thickness and the working current to gain the output without the reliability loss.