The problem proposed in this study are (1) what is the natural sciences program
students ‘vocabulary knowledge? (2) What is the natural sciences program students
‘writing skill? (3) Is there any correlation between vocabulary knowledge and
writing skill? Therefore, the aims of the study is to investigate the vocabulary
knowledge of natural sciences program students, the writing skill of natural
sciences program students, the correlation between vocabulary knowledge and
writing skill. Since the research intends to seek A Correlation Study between
Vocabulary Knowledge and Writing Skill of the Twelfth- Grade Natural Sciences
Program Students, a quantitative research was applied in this study. There are three
main data to answer the research questions. They are vocabulary knowledge data
was taken by Nation’s Vocabulary levels test (VLT), the students’ essay writing
data analyzed by Laurence Anthony’s Ant WordProfiler and to find out the
correlation between the students’vocabulary knowledge and their writing skill, the
researcher analyzed both data by using Pearson Product Moment Correlation. The
result showed us that the students have not reached intermediate to upper
intermediate proficiency level. Since the largest average value is found in the 2000
Words Level Test, which is 61.81. The average of Type Token ratio reached 0,52.
Therefore, it can be assumed that the students tend to repeat the same vocabulary
over and over again without considering for alternative variations of the vocabulary
that they used. The repeated usage of certain vocabulary causes large number of
word tokens yet small number of word types, which leads to low values of TTR.
The hypothesis of H0: r = 0 (data of Writing test score and 5000 word level test is
independent), H1: r ≠ 0 (there is correlation between Writing test score and 5000
word level test) with the Signification level α = 0,05. Test statistics r (correlation
value) = 0.342, Sig = 0.010. Decision making rules, H0 is rejected if the value of
Sig < α/2. Decision rejected H0 and received H1 since the value of Sig = 0.010 <
0.025. So, the Conclusions is there is significance correlation between two
variables, the correlation value is 0.342 (positive correlation).