The pivotal role of vegetation uptake in the global biogeochemical cycle of mercury (Hg) necessitates investigation on Hg species and transformation within plants, which have been limited by methodological constraints. This study established a temperature-programmed thermal desorption method to provide well-resolved thermal release profiles of Hg complexes with various biomolecules and Hg in plant tissues, showing significant differences from those of inorganic Hg compounds. Wild plant tissues and Hg(0)-exposed poplar leaves in the laboratory demonstrated consistency with Hg complexes with biomolecules at 180℃, 220℃, and 280℃. Besides, thermal release profiles revealed that a proportion of plant Hg is more thermally stable compared to Hg-biomolecule complexes. Specifically, for wild plants, 13%-42% of Hg in leaves was released above 300℃, and 61%-76% of Hg in roots was released between 280℃ and 450℃, likely caused by different matrices, Hg sources, and transformation processes. Results also revealed a complete transformation of Hg(0) into oxidized Hg after foliar Hg(0) uptake. The notable Hg release from 180℃ to 450℃ raises concerns about Hg emissions during various biomass thermal processes, beyond biomass burning. Therefore, controlling Hg release in these processes is promising for reducing emissions and producing low-Hg biofuels.