The objective of this study is to develop a decentralized control algorithm for attitude synchronization of spacecraft formation flying using Reaction Wheel Assembly (RWA). For the control, the absolute and relative dynamics with RWAs are formulated; these are used to construct the feedback controllers. The State-Dependent Riccati Equation (SDRE) technique is used to regulate state errors. In order to reduce the computational burden of solving the algebraic Riccati equation for relative motion, a parameter-dependent connectivity condition is suggested. The reliability of the proposed control law is validated by using a new system, which contains a Hardware-In-the-Loop Simulator (HILS). The experimental results show the effectiveness of the proposed control law.