This article presents a corpus-based investigation on English prepositions of time presented in the argumentative essays of Form 4 and Form 5 Malaysian secondary students in the MCSAW corpus. The aims were to find out the distribution patterns and the common errors in the use of preposition of time, "on" and "at". This corpus based study investigated the distribution patterns of on and at in an argumentative type of essay. Furthermore, this study also highlighted some common errors found in the use of these preposition of time and prepare some possible explanations for the errors. Last but not least, the result of this study can provide suggestions on the teaching of preposition of time on and at and also the use of MCSAW Corpus for future studies.