The problem of this research was the students had low ability in expressing their ideas. In learning activities only focused on teacher centered. As the result, the students got low value in learning process. To solve the problems, the researcher have conducted the research by using an Open Ended approach on Mathematics subject at grade X of SMAN I Nan Sabaris Kabupaten Padang Pariaman. The purpose of this research to know that student’s achievement by using an Open Ended approach was better than student’s achievement by using conventional learning at grade X of SMAN I Nan Sabaris Kabupaten Padang Pariaman. The type of this research was experimental research. The population was all of students grade X of SMAN I Nan Sabaris Kabupaten Padang Pariaman on academic year 2012/2013, except excellent class. For sampling, X.3 as experimental class and X.6 as control class. Instrument of this research was achievement test. In average, the value of experimental class was 72.38 and for control class was 63.06. After analyzing the data, the researcher found that tcalculated (2.46) was bigger than ttable (2.00). So that, the hypothesis was accepted. The conclusion of this research was student’s achievement by using an Open Ended approach was better than student’s achievement by using conventional learning at grade X of SMAN I Nan Sabaris Kabupaten Padang Pariaman. Key words : learning, open ended, achievement