The atmospheric pressure dc gliding arc plasma was investigated through its electrical and optical signals. The arc voltages of various gas discharges were compared. The arc voltage, current, power and resistance of a nitrogen gliding arc in one period were studied. Influences of gas type, gas flow rate and external resistor on the gliding arc fluctuation behaviour were studied using FFT spectrum analysis. Increased main oscillation frequency was observed as gas flow rate or external resistance value increased. Furthermore, major radical species in nitrogen, oxygen and air discharges were determined by means of optical emission spectroscopy. Additionally, the effect of external resistor on the relative intensity of radical emission and axial distribution of relative intensity at 337.1 nm (N2(C3Πu→B3Πg), Δv=0) were studied. Experimental results showed that radical emission relative intensity decreases with the increasing of external resistance value. The axial distribution of relative intensity exhibits the tendencies of increasing first and then decreasing. The radical emission relative intensity decreases dramatically out of the plasma area.