Based on a small corpus, this study investigates how “criticality”, a core requirement of academic assignment writing, is constructed by linguistic resources. The method of Rich Feature Analysis was adopted to explore how language resources are mobilized to synthesize and evaluate to achieve criticality, specifically, Theme choice, thematic progression pattern, hyper-Theme and reporting style. The analysis revealed that the stronger essay demonstrates expertise in manipulating different types of Themes, showing a clear logical pattern of Theme progression and a clear overall organization through utilization of hyper-Themes. It also mainly employs non-reporting style to enable evaluation of references. By making the linguistic realization of criticality explicit, this study helps novice writers better understand criticality, and informs writing teachers in academic writing.