ENGLISH:
This study aimed to investigate whether there was a significant difference in students' vocabulary mastery between those taught using the Word Detective Strategy and those who were not, as well as how students' vocabulary mastery differed before and after implementing the Word Detective Strategy at the seventh grade at SMP N 6 Sungaiselan. The research employed a quasi-experimental design with a pre-test and post-test control group involving 78 seventh-grade students of SMP N 6 Sungaiselan. The participants were divided into two groups: the experimental group who were taught using the Word Detective Strategy and the control group who were taught without using the Word Detective Strategy. Both groups were given a pre-test and post-test to measure their vocabulary mastery. The data were analyzed using t-tests in SPSS 22. The statistical analysis showed that the mean post-test score for the experimental group was 81.66, while the control group scored 72.13. The mean difference between the two groups was 9.46, with a standard error of 1.373. The Sig.(2-tailed) value was 0.000, and the t-obtained = 6.898, which was higher than the critical value of t-table = 2.00 at the 0.05 significance level with 76 degrees of freedom. Therefore, the null hypothesis (H0) was rejected and the alternative hypothesis (Ha) was accepted. This meant there was a significant difference in vocabulary mastery between students taught using the Word Detective Strategy and those who were not. In conclusion, the use of the Word Detective Strategy had a significant enhancement on the students' vocabulary mastery at the seventh grade at SMP N 6 Sungaiselan.