In this paper, we apply the bifurcation theory to study the steady two-dimensional periodic equatorial water waves for the f-plane approximation. We consider the waves with vorticity which propagate with a specified fixed depth between the thermocline and the upper boundary of the centre layer. Moreover, we present a functional J and its first variation corresponds to the exact equations in the transformed variables. Using the second variation of J, we prove a formal stability result for the bifurcation inducing the laminar solution. (c) 2020 Elsevier Inc. All rights reserved.