We discuss how much we can probe the effective number of neutrino species N _ { \nu } with cosmic microwave background alone .
Using the data of WMAP , ACBAR , CBI and BOOMERANG experiments , we obtain a constraint on the effective number of neutrino species as 0.96 < N _ { \nu } < 7.94 at 95 % C.L .
for a power-law \Lambda CDM flat universe model .
The limit is improved to be 1.39 < N _ { \nu } < 6.38 at 95 % C.L .
if we assume that the baryon density , N _ { \nu } and the helium abundance are related by the big bang nucleosynthesis theory .
We also provide a forecast for the PLANCK experiment using a Markov chain Monte Carlo approach .
In addition to constraining N _ { \nu } , we investigate how the big bang nucleosynthesis relation affects the estimation for these parameters and other cosmological parameters .