We correlate the failure in miniature X-ray tubes with the field emission gate leakage current of gated carbon nanotube emitters. The miniature X-ray tube, even with a small gate leakage current, exhibits an induced voltage on the gate electrode by the anode bias voltage, resulting in a very unstable operation and finally a failure. The induced gate voltage is apparently caused by charging at the insulating spacer of the miniature X-ray tube through the gate leakage current of the field emission. The gate leakage current could be a criterion for the successful fabrication of miniature X-ray tubes.