While many studies have investigated the effect of task complexity on L2 writing, little has been reported on the effects of intended task complexity manipulations on task-generated cognitive demands in L2 writing. This study, therefore, was designed to examine the relative effects of task complexity and cognitive demands on students' L2 writing. Two argumentative writing tasks were manipulated with varying numbers of elements and reasoning demands to be distinguished either as a simple or complex writing task. Self-ratings and dual-task methodology were adopted to validate the manipulations of task complexity. Thirty-one L2 learners, in the single-task group, were asked to complete two writing tasks and a post-task questionnaire. Participants in the dual-task conditions (30 in Experimental 1 and 31 in Experimental 2) were required to simultaneously complete the primary writing tasks and the secondary tasks. Results from self-ratings and dual-task experiments supported the efficacy of the task complexity manipulations.