This paper deals with the nonlinear phase field system ∂ t (θ + ℓϕ) − ∆θ = fin Ω × (0, T ),Here N ∈ N, T > 0, ℓ > 0, f is a source term, β is a maximal monotone graph and π is a Lipschitz continuous function. We note that in the above system the nonlinearity β + π replaces the derivative of a potential of double well type. Thus it turns out that the system is a generalization of the Caginalp phase field model and it has been studied by many authors in the case that Ω is a bounded domain. However, for unbounded domains the analysis of the system seems to be at an early stage. In this paper we study the existence of solutions by employing a time discretization scheme and passing to the limit as the time step h goes to 0. In the limit procedure we face with the difficulty that the embedding H 1 (Ω) ֒→ L 2 (Ω) is not compact in the case of unbounded domains. Moreover, we can prove an interesting error estimate of order h 1/2 for the difference between continuous and discrete solutions.2010 Mathematics Subject Classification: 93C20, 34B15, 35A35, 82B26.