This article addresses experiments and theories of a self-compacting gypsum-based lightweight composite (SGLC). A ß-hemihydrate is used as binder and lightweight aggregate (LWA, 0–2 mm in different size ranges) is used as aggregate into this composite. The mix of the new composite is designed based on the particle size distribution grading theory applying the modified Andreasen and Andersen (A&A) grading line [1] to obtain an optimal packing of all the used solid materials. The effect of the distribution modulus (q) in the modified Andreasen and Andersen equation is investigated. The developed mixes are studied in both fresh and hardened states, including the flowability, density, porosity, and mechanical property. The thermal behavior of the new developed composites is investigated, from its thermal physical properties, and strength degradation at elevated temperature. The study demonstrates that this new composite has significantly improved thermal and mechanical properties, compared to those of traditional gypsum plasterboard.