An atmospheric-pressure argon plasma discharge with a length of 63 cm is generated in a quartz capillary by using a pair of hollow needle electrodes. The discharge mode transition from abnormal glow to arc is investigated by means of electrical measurement and optical emission spectroscopy. The effects of the distance between two needle electrodes and the operating frequency of power supply on the voltage-current characteristics are discussed. The plasma electron density was estimated to be the order of 1014 cm-3 in the arc discharge. Moreover, the variation of gas temperature with the applied voltage is also studied, which is closely associated with the power dissipation. Furthermore, in measuring the oxygen atoms generated in argon/oxygen arc plasma discharge by optical actinometry, we found that the amount of oxygen atoms almost does not change with the rise of oxygen concentration.