500kV XLPE insulated cable with an insulation thickness of 27mm has been developed for long distance transmission lines. Basic studies on 500kV XLPE cable have been shown that contaminants in the insulation may be the factor determing electrical performance. This hypothesis is justified by the good correlation obtained between statistical estimations of the size of largest contaminant in the insulation and electrical characteristics of full-sized cables. Voltage-withstand and long-term tests have confirmed the design values for minimum breakdown stress, for AC and impulse voltage, and for the degradation coefficients.