Accurate optimized equilibrium geometries, harmonic frequencies and rotational constants of the gauche conform (C, symmetry) of hydrazine (N2H4) have been calculated employing a large series of basis sets and several ab initio methods. Our best estimate computed equilibrium geometries are = r(NN) = 1.434 A, r(NH) = 1.013 A, r(NH) = 1.010 A, theta(NNH), = 111.3degrees, theta(NNH), = 106.8degrees and theta(H,NNH) = 89.7degrees. These predictions were used as reference in the analysis of the experimental and theoretical data, and we expected that the present results could guide future investigations in this direction.