A numerical study of underexpanded axisymmetric supersonic jets is presented. The TVD finite-difference scheme is employed for the numerical calculation of Euler's equations. The numerical results are compared both with results from the theoretical shock cell spacing formula suggested by Tam and with previous experimental results. For nozzles having no divergence angle at the nozzle exit, a good agreement is obtained among the theoretical, experimental and numerical results for pressure ratios across the nozzle until the occurrence of a Mach disk in the jet. For nozzles having a divergence angle at the nozzle exit, the effects of the divergence angle, the nozzle design Mach number and the pressure ratio across the nozzle on the shock cell length are discussed and clarified.