The aspect that really crucial in English is speaking skill. Many students have learned how to speak accurately and fluently. To be able to speak English accurately and fluently the students need to have a good grammar. By having a good grammar the students can speak clearly and accurately since grammar is the rule of the language. This study aimed to find out the correlation between students grammar mastery and students speaking skill. This study used quantitative approach and correlational design to find out the correlation between two variables. The respondents of this study were 100 students from three classes batch 2017 at a private university in Yogyakarta. Purposive sampling technique was used to select the respondents out of 118 as the population. The researcher only took 118 population because of some data from students were not complete. The data were collected through documents namely the document of students’ score of English Proficiency Test (EPT) especially the second part which test the structure and written expression and the document of speaking score taken from the subject of Listening and Speaking course for freshmen. The result from the study showed that (1) the student’s grammar mastery in this private university was categorized as a low level showed by the mean score of 39 which is based on Educational Test Service (ETS), it was categorized into low level. (2) The students’ speaking skill in this private university was categorized into a very good level showed by the mean score of 56 from the 70 maximum score. (3) If there is a correlation between the two variables, the r counted should be higher than r table. In this study the r counted is 0.306 and the r table is 0.193. It means that there is a correlation between student grammar mastery and student speaking skill. The significant value of the correlation between the two variables was 0.306, which is categorized as low correlation. It means that there is low correlation between students’ grammar mastery and students speaking skill. The low correlation happened because some of students who had high level of speaking are in the low level grammar. The lowest score in grammar does not mean the student got low score too in speaking skill. Based on the result the student to improve their grammar and they should be more confident to speak English without afraid to make a mistake in grammar.