ARIA is a 128-bit block cipher, which became a Korean Standard in 2004. According to recent research this cipher is attacked by first order DPA attack In this paper, we explain a masking technique that is a countermeasure against first order DPA attack and apply it to the ARIA. And we implemented a masked ARIA for the 8 bit microprocessor based on AVR in software. By using this countermeasure, we verified that it is secure against first order DPA attack