We have investigated effects of electric fields on the yield of secondary electron emission (SEE) from the primary electron bombardment on magnesium oxide (MgO) covering vertically grown multiwalled carbon nanotubes (MWCNTs). We observe that the yield of SEE increases up to at least 22000 at a special condition. The strong local field generated by the sharp tip of vertically grown MWCNTs accelerates secondary electrons generated by primary electrons. This eventually gives rise to so called Townsend avalanche effect, generating huge number of secondary electrons in a MgO film. Emission mechanism for such a high SEE will be further discussed with energy spectrum analysis.