The aim of this study is to find out the correlation between (1) grammatical mastery and writing skill; (2) Intelligence Quotient (IQ) and writing skill; (3) grammatical mastery and Intelligence Quotient (IQ) toward writing skill.
    
This is a correlational research method. Correlational research is a kind of quantitative research. The population was class XII of SMA Negeri 3 Surakarta in academic year 2016/2017. The sample was 28 students taken by cluster random sampling. The instruments in collecting the data were test and document. The tests were used to collect the data of grammatical mastery and writing skill, while document was used to know students’ Intelligence Quotient (IQ).
The results of this research show that 1) there is positive correlation between grammatical mastery and writing skill (rxly=0.609827; 2) there is a positive correlation between intelligence quotient (IQ) and writing skill (rx2y=0.719819988; and 3) there is a positive correlation among grammatical mastery, intelligence quotient (IQ) and writing skill (R = 0.720021)

The results of this research show that grammatical mastery and intelligence quotient (IQ) have some implication on the students writing skill especially in the students’ score and the process to write a text. Therefore, grammatical mastery and intelligence quotient (IQ) can be implemented in teaching and learning of writing. An English teacher needs to arrange some activities in his lesson plan which can increase grammatical mastery and intelligence quotient (IQ) that are related to the teaching and learning of writing