With the aim of investigating effective approaches to extensive reading in Thailand, this study examined the vocabulary uptake and retention rate from reading 350 headwords graded reader. To determine the effects of reading a graded reader on students' vocabulary knowledge and retention, 20 words within four bands of different frequencies of occurrences were selected as target words in the study. The spelling of each word was changed to ensure that students have never seen these words before. 35 intermediate level Thai students volunteered to participate in the study. Three tests, which were word-form recognition, meaning recognition, and translation, were administered immediately after reading, then after 10 days, and then after a two-month delay. The results demonstrate that words can be learned incidentally, as the majority of the words were not learned from reading one graded reader. Words with more frequency were more likely to be learned and more resistant to decay. The data indicate that, on average, the meaning of only three of the 20 words will be recalled two months later. This study implies that just a little amount of new vocabulary is remembered after reading one graded reader, and that substantial amounts of reading are required to increase students' vocabulary.