Electronic Journal of Differential Equations, Vol. 2006(2006), No. 147, pp. 1-15. Title: Optimal regularization method for ill-posed Cauchy problems Authors: Nadjib Boussetila (Univ. Badji Mokhtar-Annaba, Algeria) Faouzia Rebbani (Univ. Badji Mokhtar-Annaba, Algeria) Abstract: The goal of this paper is to give an optimal regularization method for an ill-posed Cauchy problem associated with an unbounded linear operator in a Hilbert space. Key point to our proof is the use of Yosida approximation and nonlocal conditions to construct a family of regularizing operators for the considered problem. We show the convergence of this approach, and we estimate the convergence rate under a priori regularity assumptions on the problem data. Submitted February 28, 2006. Published November 27, 2006. Math Subject Classifications: 35K90, 47D06, 47A52, 35R25. Key Words: Ill-posed Cauchy problem; quasi-reversibility method; nonlocal conditions; regularizing family.