With the wide availability of mobile devices, efforts to accommodate these technological devices have been made in pronunciation training. This study aimed to investigate the effectiveness of mobile apps in improving the perception and production of the English vowels, /o/ and /ɔ/, which are difficult to discriminate and produce for Korean learners of English. The participants were 24 college freshmen enrolled in a general English course. They shadowed the model pronunciation from the application and corrected their production after instant feedback from the automatic speech recognition application. Their perception was evaluated in the preand post-tests, and the production was visually examined on the formant chart. The training was held for 5 minutes each day, for a month. The results showed that the training made positive effects on improving students’ perception of the target vowels. There was also some progress in their production although the improvement was not as noticeable as in perception. A discrepancy in the production was discovered between male and female participants with the latter performing better.