  In this paper, we consider deformations of Lie 2-algebras via the cohomology
theory. We prove that a 1-parameter infinitesimal deformation of a Lie
2-algebra $\g$ corresponds to a 2-cocycle of $\g$ with the coefficients in the
adjoint representation. The Nijenhuis operator for Lie 2-algebras is introduced
to describe trivial deformations. We also study abelian extensions of Lie
2-algebras from the viewpoint of deformations of semidirect product Lie
2-algebras.
