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A new open-loop measurement system has been built up to test the stability of metal halide (abbr. MH) lamps in this paper. Based on the new measurement system, a lot of small power MH lamps (70 W) have been tested and arrived at the followings conclusions: 1) the acoustic resonance of MH lamp is caused by itself while it operates in high frequency; 2) the stable operating frequency range is about under 10 kHz when the lamps are driven by the square-wave current; 3) the transform times, including rise-time and fall-time, of the high frequency square-wave current makes big effect on the lamp's stability. The experiments show that reducing the transform time will improve the lamp's stability; 4) with operating frequency increasing, the range of dimming would reduce to make sure it working in stable.