Abstract :
The main objective of this study is to find out how effective the portfolio is as an assessment instrument for students' procedure text writing skills. The method of this study is experimental research. Data obtained by giving tests to the experimental class and the control class after giving different treatments for the two classes. From the results of the study found the average score of the experimental class writing (assessed using a portfolio) was 77.50 and the average score of writing the control class (students assessed using non-portfolio) was 78.43. There was no significant difference in the writing procedure text scores between students who were assessed using portfolios and those who did not. That is indicated by the average experimental class higher than the control class. On the other hand, hypothesis testing using the t-test formula shows a lower t-value than t-tables. The t-value is 1.94, while the t-table at = 5% with df 90 is 1.66 (1.30> 1.66). So, the hypothesis is not accepted. Based on the results of the study, it can be concluded that the portfolio as an effective assessment instrument for students' writing skill of procedures text.