Granular materials are widely spread in nature and in industry. Owing to the inelastic collisions between particles and frictions among particles, granular systems are dissipative in nature. This intrinsic dissipative nature causes local clustering in granular gas systems. This is a unique phenomenon compared with the molecular gases. Understanding and predicting the condition and parameter values when this phenomenon happens will be helpful for us to gain knowledge of the conditions of clustering or pattern formations in non-equilibrium complex systems. The clustering phenomenon in granular gas is analyzed using phase-separation modeling of van der Waals-like molecules. The results from the model are verified by molecular dynamics numerical simulations. However, due to the influence of the gravity, experimental verification is difficult in laboratory. In this work, we perform an experiment in micro-gravity environment provided by the drop tower of National Microgravity Laboratory Chinese Academy of Science. In the experiment we for the first time observe the phase-separation clustering phenomenon. Comparing the observation condition with the model prediction, we are able to indirectly obtain the restitution coefficients of particles used in the experiment. A model calculation for the spinodal regime under experimental conditions is performed for possible particle restitution coefficients, and a comparison with the experimental observation allows us to justify the values of the restitution coefficients. It is found that the coefficient is larger for bigger particles. For d=2.5mm titanium particles, the restitution coefficient is higher than 0.8; for d=1mm titanium particles, the restitution coefficient is about 0.8, and for d=0.5mm titanium particles, the restitution coefficient is between 0.6 and 0.8. This useful result can be essential for comparing experimental observation with the theoretical and the numerical results, and is crucial to the success in the SJ-10 satellite experiments.