A new live-line insulator tester was developed to detect faulty insulators in 154kV power transmission lines. This paper is the first part of the two-part paper and deals with its tester development. The developed tester has the following characteristics. First, it automates parts of the insulator inspection process by using a sensor and an actuator, and secondly, it sustains its weight by itself, not by a lineman. Finally, it measures the insulation resistance of an insulator together with its distribution voltage to provide more information for its analysis and diagnosis. These leads to the improvement of its operation efficiency, measurement reliability and usability. Its effectiveness was validated by live-line field tests in actual power lines.