The thermal behaviour of the Rosa multiflora (3 groups: control, irradiated 3 × 3 seconds, and 3 × 9 seconds by low power laser of wavelength 672 nm) and Miscanthus giganteus (3 groups: R, V, and Z) biomass by thermogravimetric analysis (TGA) was studied at heating rate 3 K per minute from ambient temperature to 950°C. The TGA tests were performed in high purity CO2 (99.998%) with a flow rate 200 mL per minute and 100 mg of sample, milled and sieved to a particle size below 250 μm. In contrast to the majority of the conducted examinations, which are carried out with the use of chars, the measurements were conducted for raw biomass. The kinetics parameters of the biomass conversion process were determined from the experimental data by integral methods (Murray and White; Senum and Yang). The methods used have allowed distinguishing and analysing the different stages of the process, i. e. the primary and secondary pyrolysis as well as gasification. Both methods gave comparable results. The activation energy determined using more accurate Senum and Yang method ranged from 85-88 kJ/mol (primary pyrolysis), from 29-32 kJ/mol (secondary pyrolysis), and from 173-190 kJ/mol (gasification) for Rosa multiflora biomass and from 86-111 kJ/mol (primary pyrolysis), from 22-25 kJ/mol (secondary pyrolysis), and from 181-217 kJ/mol (gasification) for Miscanthus giganteus. The pre-exponential factor was in the range of 5.95 ⋅ 106 to 13.05 ⋅ 106 min–1 (primary pyrolysis), 2.25-4.22 min–1 (secondary pyrolysis), and 13.96 ⋅ 106 to 1.18 ⋅ 108 min–1 (gasification) for Rosa multiflora biomass and 6.94 ⋅ 106 to 1.79 ⋅ 109 min–1 (primary pyrolysis), 0.88-1.62 min–1 (secondary pyrolysis), and 131.54 ⋅ 106 to 1.92 ⋅ 109 min-1 (gasification) for Miscanthus giganteus biomass.