The anomalous Nernst effect (ANE), generating spin thermoelectric signal (E) through spin-orbit coupling, is an important mechanism to explore the interaction between charge, heat, and spin. In this work, we employ the longitudinal experimental setup with a uniform out-of-plane temperature gradient (▽ T) in various ferromagnetic materials (FMs), including Fe, Co, Ni, and Py (Ni80Fe20), with in-plane anisotropy to study the ANE. The magnitude and sign of the ANE exhibit nontrivial thickness dependent behaviors which do not simply follow the behaviors of the saturation magnetization (Ms) and resistivity (ρ). While the sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, it can even be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles (θANE), the conversion efficiency of the spin/charge signal, for these FMs can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms in thin film.