A uniform plasma plume was generated in a coaxial dielectric barrier discharge jet through blowing argon into the ambient air at atmospheric pressure. The plasma plume was uniform along the direction of the gas flow. The length of the plasma plume was investigated as a function of the peak voltage, the driving frequency and the gas flow rate. It was found that with increasing the gas flow rate, the plume length increases when the flow rate is lower than 4 L·min-1, and decreases when it is higher than 4 L·min-1. Under constant gas flow rate, the length of the plasma plume increases with the increase in the peak value of the applied voltage and the driving frequency. According to the discharge theory and based on the analysis of the turbulence and the advection, a qualitative explanation was given for the variance of plume length as functions of the experimental parameters. Results also show that there is a discharge pulse for the plasma plume in every positive half cycle, while there is no pulse in negative half cycle. The coaxial dielectric barrier discharge shows two pulses in every positive half cycle and a pulse in every negative half cycle. Analyzing these experimental phenomena mentioned above, a formation mechanism of the plasma plume was proposed. The optical emission spectra were obtained for both the coaxial dielectric barrier discharge and the plasma plume. There was no apparent difference except that some emission lines from reactive species such as OH and N2 were found in the plasma plume. Using the first negative band of, the rational temperature of the plasma plume was measured. Results show that the rational temperature of the plasma plume decreases away from the jet nozzle, and increases with increasing the peak value of the applied voltage.