The paper seeks to investigate the level of productive knowledge of ESL learners, the writing quality and the relationship between the vocabulary knowledge and the writing quality. 150 final year students of English language in a university in Nigeria were randomly selected as respondents. The respondents were asked to write an essay of 300 words within one hour. The essays were typed into Vocab Profiler of Cobb (2002) and analyzed the Lexical Frequency Profile of the respondents. The essays were also assessed by independent examiners using a standard rubric. The findings reveal that the level of productive vocabulary knowledge of the respondents is limited. The writing quality of the majority of the respondent is fair and there is a significant correlation between vocabulary and the witting quality of the subjects. The researchers posit that productive vocabulary is the predictor of writing quality and recommend various techniques through which teaching and learning of vocabulary can be improved.

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