Isoconversional methods are commonly used to process the thermogravimetric analysis (TGA) data and to simultaneously obtain the effective activation energies for lignocellulosic biomass pyrolysis. However, the widely used isoconversional methods may lead to some systematic problems, for example, numerical instability for the Friedman method and significant errors in the kinetic parameters for the Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods. To avoid the above problems, a modified Friedman isoconversional method has been developed in this work to accurately determine effective activation energies for solid-state reactions, such as biomass pyrolysis reactions. Through processing theoretically simulated data of strongly varying activation energy, the modified method applying for narrow intervals of conversion degree was proven to be capable of calculating the conversion dependence of activation energy accurately and reducing the effect of data noise. The modified Friedman isoconversional method was employed to process the non-isothermal TGA data of wheat straw pyrolysis at heating rates of 2.5, 5, 10, and 20 K min(-1) and the kinetic data of beech sawdust pyrolysis at 5, 10, and 20 K min(-1) from the literature. The results showed that the effective activation energies for the pyrolysis of wheat straw and beech sawdust varied significantly with the degree of conversion (in the conversion range from 0.05 to 0.85, the effective activation energies for wheat straw pyrolysis varied from 154 to 379 kJ mol(-1), while 155-316 kJ mol(-1) was reported for beech sawdust pyrolysis).