The oscillation of the CMB photons into axions can cause CMB spectral distortion in the presence of large scale magnetic field .
With the COBE limit on the \mu parameter and a homogeneous magnetic field with strength B \lesssim 3.2 nG at the horizon scale , an upper limit on the axion mass is found to be , m _ { a } \lesssim 4.8 \times 10 ^ { -5 } eV for the KSVZ axion model .
On the other hand , using the value of excluded axion mass m _ { a } \simeq 3.5 \times 10 ^ { -6 } eV from the ADMX experiment together with the COBE bound on \mu , is found B \simeq 46 nG for the KSVZ axion model and B \simeq 130 nG for DFSZ axion model , for a homogeneous magnetic field with coherence length at the present epoch \lambda _ { B } \simeq 1.3 Mpc .
Forecast on B and m _ { a } for PIXIE/PRISM expected sensitivity on \mu are derived .
If CMB \mu distortion would be detected by the future space missions PIXIE/PRISM and assuming that the strength of the large scale magnetic field is close to its canonical value , B \simeq 1 - 3 nG , axions in the mass range 2 \mu eV - 3 \mu eV would be potential candidates of CMB \mu -distortion .