The dispersion characteristics of surface plasmon polaritons (SPPs) in the terahertz frequency range are studied in ordered arrays of single- and double-walled carbon nanotubes immersed in a dielectric medium. Dispersion relations for SPPs in an isolated nanotube with a dielectric environment were obtained analytically and then used to verify the solutions obtained by numerical simulation. Frequency dependences of the propagation and attenuation constants, as well as the SPP deceleration coefficient, are obtained for arrays of different densities by numerical simulation. It is found that in dense CNT arrays, the propagation constants and the mean free path of SPPs are smaller than in isolated nanotubes. The deceleration coefficient increases with increasing nanotube diameter and permittivity of the host medium. Keywords: single- and double-walled carbon nanotubes, dense nanotube array, surface plasmon polaritons, surface wave deceleration.