Recently, there is a growing interest in the spectral approximation by the Prolate Spheroidal Wave Functions (PSWFs) ψ n,c , c > 0. This is due to the promising new contributions of these functions in various classical as well as emerging applications from Signal Processing, Geo-physics, Numerical Analysis, etc. The PSWFs form a basis with remarkable properties not only for the space of band-limited functions with bandwidth c, but also for the Sobolev space H s ([−1, 1]). The quality of the spectral approximation and the choice of the parameter c when approximating a function in H s ([−1, 1]) by its truncated PSWFs series expansion, are the main issues. By considering a function f ∈ H s ([−1, 1]) as the restriction to [−1, 1] of an almost time-limited and band-limited function, we try to give satisfactory answers to these two issues. Also, we illustrate the different results of this work by some numerical examples.