MgO/XLPE composites, composed of micro-and nanosized MgO material having different particle size as inorganic filler and cross linked polyethylene (XLPE), were prepared and investigated to the space charge dynamics of these composites for high voltage DC insulation. Added MgO material, having 50nm to 1μm in size, were synthesized from the calcination of Mg (OH)₂nanoplatelets which was controlled by potassium content in Mg (OH)₂, varying the dilution volume ratio of distilled water/ Mg (OH)₂suspension. MgO/XLPE composites were obtained by thoroughly mixing LDPE and dicumyl peroxide (DCP) as the cross-linking agent and silane modified MgO before kneading to enhance the dispersion degree of the composites. The space charge distributions of the composites were observed by controlling the particle size and content of a dispersed MgO in XLPE matrix. To prevent the breakdown under high DC stress, it was found that the reduction of space charge of MgO/XLPE composite depends upon the smaller size of MgO particle and the its content above 0.5 phr (per hundred part of resin, wt%). MgO material and MgO/ XLPE composites were characterized with TEM, XRD, BET, ICP, and the space charge properties of the composites were measured by a pulsed electro-acoustic method (PEA).