The Epoxy composite foam (ECF) composed by hollow glass microballoon (HGMB) and epoxy matrix has a good application potential as an alternative solid insulation material with light weight and good temperature adaptability. In this paper, the effect of volume fraction of HGMB on two critical dielectric properties of ECF, namely breakdown field strength (EBD) and relative dielectric constant εr, was studied. It was found EBD of ECF under pulse and AC voltage did not change with the volume fraction of HGMB, and the effect of HGMB volume fraction on EBD under pulse voltage was smaller than that under AC voltage; the longer withstanding time and the lower EBD of epoxy matrix under AC voltage than those under pulse voltage were responsible to the difference of breakdown characteristics. The εr of ECF decreased with the increase of HGMB volume fraction, and it decreased with the increase of frequency at various temperatures (−60 ℃ ~ 90 ℃), but the change trends at high and low frequency regions with temperature were different; by comparing the results of pure epoxy, it was speculated that the change trend of εr of ECF with temperature in high frequency region was mainly determined by epoxy matrix; while in low frequency region, it was mainly determined by epoxy matrix at middle and high temperature regions and by epoxy matrix at low temperature region.