It was shown that in mechanoactivated states of manganese oxides the following effects can be explained with application of Jahn-Teller effect theory: - narrowing of stability range for the cooperative Jahn-Teller phase y-Mn304 (hausmannite); - the above phase disappears for the grain size of the order of 10 nm because of the a- Mn203 phase reduces directly to the spinel phase P-Mn304; - the transition from metastable y-Mn304 to stable a-Mn203 at heating of nano-size Mn304 from room temperature can takes place at temperatures distinctly lower than for coarsegrained samples.