In this paper, we investigate Lagrangian submanifolds in the homogeneous nearly Kähler S3 × S3. We introduce and make use of a triplet of angle functions to describe the geometry of a Lagrangian submanifold in S3 × S3. We construct a new example of a flat Lagrangian torus and give a complete classification of all the Lagrangian immersions of spaces of constant sectional curvature. As a corollary of our main result, we obtain that the radius of a round Lagrangian sphere in the homogeneous nearly Kähler S3 × S3 can only be 2/√3 or 4/√3.