In this talk, among many useful aspects of anisotropy in temperature and polarization of Cosmic Microwave Background (CMB), we focus on how it can allow us to probe a dark radiation species of the early universe that does not belong to the Standard Model. To this end, we shall study imprints of perturbation in matter sector on CMB power spectrum, based on cosmological perturbation theory. And then we discuss how one can constrain the amount and decoupling time of the dark radiation from the imprints. Lastly, as an example of application, we will show how one can constrain a BSM model (Nnaturalness model) using the imprints of CMB power spectrum.