Defects may probably appear during the lifetime of HVDC cable, and possibly distort the electric field distribution of HVDC cable and further accelerates insulation degradation. Two-dimension and three-dimension finite element analysis (FEA) are applied to study the influence of single spherical defect on the surrounding dielectric under differernt conditions including different temperature difference level across insulation, different defect location and size. A general equation for describing field distortion by spherical defect in HVDC cable is given in the paper, and it is concluded that three-dimension analysis is advantageous in researching the defect distortion, electric conductivity is the key factor in determining the electric field enhancement by defect and the influence of defect on surrounding insulation material is limited in a small region.