SonicPrint™, a free 2D real-time visual feedback software for windows was adjusted and installed in a CALL room at a university in Japan and applied for English vowel pronunciation training with a total of 97 students for four times. The vowel sounds produced by the students were recorded on three occasions, pre-training, post-training, and one month after the post-training. The formant frequency measurements of eight randomly-selected students were analysed and mapped on F1-F2 vowel charts. Although a powerful L1 vowel influence was observed, the vowel training software has shown promising results.