Four Square Writing is a technique to improve, help, connect, and organize students' thought in writing. It also helps the students generalize and combine the best outline, fact and ideas. The steps are categorizing the relationship, labeling the topic sentence, giving more details, writing paragraph, making summary, and adding or connecting transition word. This article refers to an experimental research about the effectiveness of Four Square Writing in teaching writing at a senior high school. The population of the research is the eleventh grade students of a senior high school, in Karanganyar. The sample of the research consist of two classes; IX Science 1 was used as experimental class treated by using Four Square Writing and IX Science 2 was used as control class treated by using Guided Writing. The sampling technique used is cluster random sampling. The instruments used to collect the data are students' creativity test and writing test. The data were analyzed by using multifactor analysis of variance (ANOVA 2X2) and Tukey test. Based on the result of the analysis, the findings of the research are: (1) Four Square Writing is more effective than Guided Writing to teach writing; (2) the students having high creativity have better writing skill than those having low creativity; and (3) there is an interaction between teaching techniques and students' creativity to teach writing. In conclusion, Four Square Writing is an effective technique to teach writing for eleventh grade students of a senior high school, Karanganyar. The effectiveness of technique is influenced by the students' level of creativity.