In this article, we address the Cauchy problem for the KP-I equationfor functions periodic in y. We prove global well-posedness of this problem for any data in the energy spaceWe then prove that the KdV line soliton, seen as a special solution of KP-I equation, is orbitally stable under this flow, as long as its speed is small enough.From the work of Benjamin [2], we know that these solutions are orbitally stable in H 1 (R) under the flow generated by the KdV equation (1.2), meaning that every solution of (1.2) with initial data close to Q c in H 1 (R) remains close in H 1 (R) to the Q c -orbit (under the action of translations) at any time t > 0.Looking at (1.1), we see that every solution of the KdV equation (1.2) is a solution of the KP equations (1.1), seen as a function independent of y. It is then a natural question to ask whether Q c is orbitally stable or unstable under the flow generated by (1.1). In order to do so, we first need a global well-posedness theory for (1.1) in a space containing Q c . In particular, this rules out any well-posedness result in an anisotropic Sobolev space H s1,s2 (R 2 ). A more suited space to look for is the energy space for functions periodic in y :where T = R/2πZ. Indeed, due to the Hamiltonian structure of (1.1), the mass M(u)(t) := R×T u 2 (t, x, y)dxdy (1.4) and the energy E(u)(t) := R×T (∂ x u) 2 (t, x, y) + (∂ −1 x ∂ y u) 2 (t, x, y) −