The present study aims to investigate the effectiveness of Research Based Learning (RBL) in improving students' learning achievement in solving two dimensional arithmetic sequence problems. The study applied triangulation method including qualitative and quantitative methods. The research subjects were 4th-semester students of higher education. The quantitative method was applied to analyse the learning achievement test, while the qualitative one was applied in the form of observation and interview. The collected data were analysed by using inferential statistics t-test. The result of t-test indicates that the sig (2-tailed) of independent sample t-test of pre-test is 0.853 (p>0,05), thus it is not significant. It implies the two classes are homogeneous in term of student achievement test. The data analysis of independent sample t-test of post-test, the sig (2-tailed) is 0.00 (p=<0,05), thus it is significant. It implies that the student achievement tests of two classes are different after the implementation of RBL. It shows that the students' achievement result of experimental class is better than the control class. The implementation of RBL is proven effective in improving students learning achievement in solving two-dimensional arithmetic problems.