  We studied a constraint on the primordial helium abundance Y_p from current
and future observations of CMB. Using the currently available data from WMAP,
ACBAR, CBI and BOOMERANG, we obtained the constraint as Y_p =
0.25^{+0.10}_{-0.07} at 68% C.L. We also provide a forecast for the Planck
experiment using the Markov chain Monte Carlo approach. In addition to
forecasting the constraint on Y_p, we investigate how assumptions for Y_p
affect constraints on the other cosmological parameters.
