In this paper, we consider the usual linear regression model in the case where the error process is assumed strictly stationary. We use a result from Hannan, who proved a Central Limit Theorem for the usual least squares estimator under general conditions on the design and the error process. We show that for a large class of designs, the asymptotic covariance matrix is as simple as the independent and identically distributed case. We then estimate the covariance matrix using an estimator of the spectral density whose consistency is proved under very mild conditions. 1 independent and identically distributed. 1 (ǫ i ) is satisfied, we propose a consistent estimator of the spectral density of (ǫ i ) (as a byproduct, we get an estimator of the covariance series).Wu and Liu [14] considered the problem of estimating the spectral density for a large class of short-range dependent processes. They proposed a consistent estimator for the spectral density, and gave some conditions under which the centered estimator satisfies a Central Limit Theorem. These results are based on the asymptotic theory of stationary processes developed by Wu [23]. This framework enables to deal with most of the statistical procedures from time series, including the estimation of the spectral density. However the class of processes satisfying the L 2 "physical dependence measure" introduced by Wu is included in the class of processes satisfying Hannan's condition. In this paper, we prove the consistency of an estimator of the spectral density of the error process under Hannan's condition. Compared to Wu's precise results on the estimation of the spectral density (Central Limit Theorem, rates of convergence, deviation inequalities), our result is only a consistency result, but it holds under Hannan's condition, that is for most of short-range dependent processes.The paper is organized as follows. In Section 2, we recall Hannan's Central Limit Theorem for the least squares estimator, and we define the class of « regular designs » (we also give many examples of such designs). In Section 3, we focus on the estimation of the spectral density of the error process under Hannan's condition. Finally, some examples of stationary processes satisfying Hannan's condition are presented in Section 4.Theorem 2.1 is very general because it includes a very large class of designs. In this paper, we will focus on the case where the design is regular in the following sense: Definition 2.3.1 (Regular design). A fixed design X is called regular if, for any j, l in {1, . . . , p}, the coefficients ρ j,l (k) do not depend on k.A large class of regular designs is the one for which the columns are regularly varying sequences. Let us recall the definition of regularly varying sequences: Definition 2.3.2 (Regularly varying sequence [21]). A sequence S(·) is regularly varying if and only if it can be written as:where −∞ < α < ∞ and L(·) is a slowly varying sequence. 2 The transpose of a matrix X is denoted by X t .