A novel method is presented for the determination of mode I cohesive law for the characterization of delamination in unidirectional carbon fiber reinforced polymer laminates. The energy release rate as a function of the crack advance is determined based on an analytical approach where the compliance variation of the specimen as crack advances is used in order to obtain the crack length for every pair of load and displacement data. Based on the same analytic approach, the crack tip opening displacement is determined as a function of the equivalent crack advance assuming that the Fracture Process Zone development is analogous to an equivalent crack advance Δa. These measurements are used to compute the Energy Release Rate and crack opening displacement from which a cohesive law is determined by numerical differentiation. This new method provides a simple way to obtain the mode I cohesive law using only the load and displacement data obtained from the testing machine, without any external displacement measurement technique and without any assumption of the form of the cohesive law.