This study attempted to develop computer-based English diagnostic tests in five language skill areas, (i.e., listening, reading, writing, grammar and vocabulary), to administer the tests to English learners of an online university in Korea, and to provide each participant with diagnostic feedback on his/her strengths and weaknesses based on the results. The detailed procedure and output of the study are as follows. First, a list of language subskills or achievement standards was created for each language skill area based on a detailed analysis of the English curriculum of the university and experts’ discussions. Next, based on the test specification prepared, test items were developed to measure the target subskills. Third, the developed test items were uploaded to an online test system and administered to those who enrolled in English courses offered in the 2nd semester of 2016. Fifth, each test taker’s performance on the test was scored and analyzed in order to diagnose his/ her strengths and weaknesses in terms of the subskills of each language skill. Sixth, each test taker was provided with detailed feedback on their strengths and weaknesses, regarding not only which subskills they have achieved and not achieved, but also which courses they should take to supplement their weaknesses. Lastly, a questionnaire was created and administered in order to investigate the participants’ satisfaction levels and opinions on the diagnostic tests and the individual feedback that they received. The collected opinions will be reflected in future revisions of the tests.