Short-period variation of filament diameter was measured during melt-spinning of mesophase pitch using Laser-beam scanning diameter-measuring equipment. Filament diameter has been changing continuously owing to change of temperature of atmosphere under the nozzle. Frequency of oscillation of filament diameter was 10-20/s for a small disodering of temperature of atmosphere. Coefficient of variation of filament-diameter variation can be reduced when spinning is done in high viscosity or in high spinning velocity. But the low strength of pitch fiber almost limit to select such a mean.