In this article, spherical black spots-like selenium metal nanoparticles were synthesized. Accordingly, this experimental work proposed an innovative facile, green, one-step and solvent-free strategy to a large scale synthesis of Se-NPs via thermal decomposition of green precursor. The Se(Ⅳ) L-glutamine precursor was prepared by solid state grinding using selenium(Ⅳ) tetrachloride, SeCl₄, and L-glutamine for 2 hr without using any organic solvent. It was characterized by infrared spectroscopy, and micro analytical. The solid precursor compound was subsequently annealed in the muffle furnace at 300 ℃ for 3 hr in static air. Selenium NPs was resulted and well characterized using X-ray powder diffraction (XRD), FT-IR spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The FTIR and XRD data showed that the Se NPs is pure and has a good crystalline structure because no characteristic peaks of impurity were detected, while the SEM and TEM results showed that the obtained product is tiny, aggregated with spherical-like shape, narrow size distribution with an average size between 5~10 nm. Results show that the solid state thermal decomposition method is simple, eco-friendly, safe and suitable for preparation of SeNPs. This method can also be applied to synthesize nanoparticles pure metal and metal oxides.