Abstrak This paper presents the findings from a quasi-experimental one group time-series design involving a number of Pharmacy Departement students of Sanata Dharma University taking Indonesian Language subject in 2005-2006 academic year. The experiment was conducted to examined the effectiveness of a learning model to improve students’ ability in argumentative writing and critical thingking skill based on the Toulmin’s theory of argument. Facione’s critical thingking theory, and jurisprudential inquiry frame of Joyce and Weil. The instrument of this study are: a set of argumentative writing tasks, questionair, interview, students’ documents, and an observation sheet of students activities during the lesson. The study found that using t-test and content analysis techniques at 95% level of significance: Toulmin’s learning model was effective to improve students’ arguments writing and students’ critical thingking skill.