This research was conductedto find out whether there is a correlation between (1) grammar mastery and writing skill; (2) reading habit and writing skill; and (3) grammar mastery, reading habit simultaneously, and writing skill.
In this research, the correlational method was used and it was carried out in September 2014 at SMA Negeri 3 Sukoharjo. The population was  all of the second grade students of SMA Negeri 3 Sukoharjo in academic year 2014/2015, which consisted of ten classes. The number of population was 352 students and this research took 36 students as the sample by cluster random sampling. A questionnaire was used to collect the data of students’ reading habit, whilean objective test was used to collect the data of grammar mastery and essay test was used to collect data of writing skill. Single and Multiple Linear Regression and Correlation were used to analyze the data.
The result of the analysis shows that (1) there is a positive correlation between grammar mastery and writing skill (rx1y= 0.682 ); (2) there is a positive correlation between reading habit and writing skill (rx2y=0.423); and (3) there is a positive correlation between grammar mastery, reading habit simultaneously, and writing skill (ry12=0.681). Thus, it cannot be neglected that grammar mastery and students’ reading habitcontribute to writing skill.
Keywords: correlation, grammar mastery, reading habit, writing skill