We present the exact equation for evolution of Bianchi I cosmological model, considering a non-tilted perfect fluid in a matter dominated universe. We use the definition of shear tensor and later we prove it is consistent with the evolution equation for shear tensor obtained from Ricci identities and widely known in literature [3], [5], [9]. Our result is compared with the equation given by Ellis and van Elst in [3] and Tsagas, Challinor and Maartens [5]. We consider that it is important to clarify the notation used in [3], [5] related with the covariant derivative and the behavior of the shear tensor.