We focus on the ongoing and future observations for both the 21 cm line and the CMB B-mode polarization produced by a CMB lensing , and study their sensitivities to the effective number of neutrino species , the total neutrino mass , and the neutrino mass hierarchy .
We find that combining the CMB observations with future square kilometer arrays optimized for 21 cm line such as Omniscope can determine the neutrino mass hierarchy at 2 \sigma .
We also show that a more feasible combination of Planck + Polarbear and SKA can strongly improve errors of the bounds on the total neutrino mass and the effective number of neutrino species to be \Delta \Sigma m _ { \nu } \sim 0.12 eV and \Delta N _ { \nu } \sim 0.38 at 2 \sigma .