Based on Robinson's (2005) Cognition Hypothesis and Skehan and Foster's (2001) Limited Attentional Capacity Model, the current study attempted to investigate the effect of manipulating task complexity on argumentative writing quality in terms of lexical complexity, fluency, grammatical accuracy, and syntactic complexity. Task complexity was manipulated through applying resource-dispersing dimensions. All 60 participants who were university students were randomly assigned into one of the three groups: (a) topic; (b) topic + idea; and (c) topic + idea + discourse marker group. A series of one-way ANOVAs was utilized to detect significant differences among the groups. Results showed that increasing task complexity: (1) did not lead to differences in lexical complexity (measured by the ratio of lexical words to function words and lexical density), but it did lead to significant differences when mean segmental type-token ratio was used to measure lexical complexity; (2) produced significantly less fluent language; (3) resulted in more grammatically accurate language in the least complex task; and (4) did demonstrate significant difference in syntactic complexity (when it was measured by the ratio of dependent clauses to total clauses). Further findings and implications are discussed in the paper.