Theoretical calculations of the magnitude and temperature variation of the measured thermal conductivity of undoped and doped GaAs nanobeams will present. The calculations have been performed by employing modified Callaway’s theoretical model. In the model, both longitudinal and transverse modes are explicitly taken into account. Scattering of phonons is assumed to be by nanobeam boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. A method is used to calculate the Debye temperature and phonon group velocities for undoped and doped nanobeams from their related melting points. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. The drop in thermal conductivity of doped nanobeams compared to that of the undoped beams arises from electron-phonon scattering and additional phonon scattering from a large number of point impurities due to the presence of dopant atoms. Effect of Gruneisen parameter, surface roughness, and dislocations are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves.