Stable stripe pattern is observed in flowing argon at atmospheric pressure by using a dielectric barrier discharge device with two transparent water electrodes. Based on the photography and the electrical measurement, the formation mechanism of stripe is investigated. Results show that a stripe pattern can be obtained at a lower peak value of the applied voltage in flowing argon, and the discharge turns homogeneous at a higher voltage. Results show that the formation of stripe pattern results from the movement of discharge filament in the direction of gas flow. The moving velocity of filaments almost keeps constant during the voltage varying. However, the moving velocity increases with the increase of gas flow rate. The memory effect of active particles in the discharge space is very important for the formation of stripe pattern. Furthermore, the electric characteristics of discharge are studied in flowing gas in this paper. It is found that both the discharge current and the gas inception voltage decrease with the increase of the gas flow rate. A qualitative explanation is given for this experimental phenomenon. These results are of great importance for the research of pattern formation dynamics and industrial applications of dielectric barrier discharge.