This type of research is qualitative research that aims to describe and analyze the effect of grammatical understanding on the effectiveness of sentences in student thesis writers at Semarang University. The sample of this research was 70 students from four socio-economic group study programs, which is the undergraduate program of Law, Management, Accounting, and Communication Science. The data source for text analysis was obtained from four theses representing those four study programs. Based on the analysis of the completed questionnaires (70 pieces), overall, 58.22% understood and even 21.82% strongly understood, did not understand as much as 19.68%, and only 0.35% did not very understood of 4 grammatical criteria. The effect of this understanding supports the analysis of the thesis document with the findings of 730 sentences consisting of 636 sentences from the thesis writer and quotes from other people's opinions (94 sentences). It is found that 459 over 636 sentences are grammatical sentences and 177 ungrammatical sentences. The ungrammaticality of the sentence includes sentences without subject, without a predicate, subject and predicate, double subject, double predicate, object preceded by a preposition, subject, and double predicate, repeating the subject, without the object, and inappropriate use of the conjunction maka in front of the subject. The result showed 72,170% of students of Semarang University had the ability to write a grammatical sentence, meanwhile, 27, 830% of students needed to improve their ability to produce the grammatical sentence so that they resulted in a qualified thesis, both its content and its language were sophisticated.