Writing argumentative essays becomes a challenge for both teachers and students as it\ud
relates to various aspects. One of them is on the teaching and learning of critical thinking skills comprising the awareness to avoid logical flaws. The real reflection of critical thinking can be traced through students’ ability to express their thought in the form of arguments which have no logical flaws. Good arguments must be supported by convincing claims and careful choice of wordings which is free from fallacious statements. This paper identifies the fallacious statements or logical flaws occurring in the argumentative writing of the students of an Islamic University in Indonesia. The\ud
findings show the faulty reasoning found in terms of the discussion on the topics, the flawed relations with the audience, and the relations with the characters involved in the arguments. The types of the logical flaws also concern the claims expressed. On claims of fact, the fallacies found include hasty generalization, irrelevancy, slippery slope, oversimplification and begging the question. Regarding pathos, the fallacy type covers ad populum, appeal to emotion premises and red herring. Meanwhile, the ethos fallacy occurs in straw man only. On claims of value, more faulty reasoning is found\ud
compared to the discussion on the topics which are considered less familiar. In the logos fallacy for instance, the whole types of fallacy are found. The pathos found involves appeal to emotion premises and red herring, while the ethos fallacy occurs in appeal to authority and ad hominem. On claims of policy, the similar tendency of ethos is also found while the pathos existing is in the form of appeal to emotion premises. Therefore, it is recommended that the students develop their critical thinking skills which involve constructing logical and flawless reasoning