  We present measurements of the Electronic Spin Resonance (ESR) of Nitrogen
Vacancy (NV) centers in diamonds that are levitating in a ring Paul trap under
vacuum. We observe ESR spectra of NV centers embedded in micron-sized diamonds
at vacuum pressures down to $2\times 10^{-1}$ mbar, limited by the green laser
heating of the diamond. We also observe the NV photoluminescence at vacuum
pressures down to $10^{-2}$ mbar and discuss the steps required to control the
NV spin under ultra-high vacuum.
