Na ion deintercalation from alpha-NaFeO2 by charging a Li/alpha-NaFeO2 cell gives a new monoclinic phase, Na0.5FeO2. The room-temperature Mossbauer spectrum shows two kinds of doublet lines with almost same intensity. The isomer shifts are 0.33 and -0.07mm/s (relative to Fe metal), respectively, the former comes from Fe3+ state and the latter clearly originates in Fe4+ state. The trial of further deintercalation of Na ion brings the decomposition of the electrolyte and electrode. Discharge in Li/Na0.5FeO2 cell is possible, but the Na ions remained hinders smooth lithium intercalation.