The objective of this research was to find out the effectiveness of concept map to
organize ideas in writing exposition text. This research was conducted in one senior
high school in Purwokerto in academic year 2018/2019. The population was all the
eleventh grade students in that school. Researcher used two classes as control group
and experimental group. This research used quasi-experimental research which was
done in November 2019. The instrument of collecting data was test. The test was
done through pre-test and post-test. The type of the test was writing test. T -test
formula was used to analyze the data. Based on the computation, t-test result was
3.535 and the t-table was 1.677 at significant level 0.05 with the degree of freedom
48. It showed that t-result was higher than t-table. Based on the research result, it can
be concluded that concept map was effective to organize ideas in writing exposition
text.