Giant reed (Arundo donax L.), a perennial plant with a lignocellulosic structure, has become increasingly important in bioenergy production in recent years. Due to its wide natural and cultivated distribution worldwide, high biomass yield, and calorific value comparable to that of wood and other energy crops, giant reed represents a promising raw material for bioenergy production. For biomass production, it is essential in terms of raw material continuity that the product be harvested in a short time. Consequently, this study aimed to determine the chemical and thermal characterization and calorific values of naturally grown giant reed obtained at different harvest times (6th month - late summer, 9th month - autumn, and 12th month - winter). The chemical properties were found to differ according to the harvest season. The thermal characters of samples were investigated by thermogravimetric analysis (TG) both in air and under an inert atmosphere. It was revealed that the thermal character of the giant reed was similar in autumn and winter harvest seasons. Fourier transform infrared (FTIR) analysis was performed to investigate the changes that may occur in the chemical structure of the giant reed during this six-month period. Higher heating values (HHV) of the giant reed harvested at different times was found experimentally as 17 583 kJ/kg - 18 822 kJ/kg - 18 523 kJ/kg, respectively. The ash contents were determined as 5.72% - 4.59% - 4.48% during the harvest periods.