Foreign language aptitude is defined as one's potential to learn a second language. A language learner with higher aptitude is predicted to learn more, faster, and reach a higher level of proficiency. If this is the case, one way to validate the construct of aptitude and its measure is to conduct a validation study in which measures of aptitude is correlated with a learning outcome. This study aimed to conduct a concurrent validity of LLAMA_F, a foreign language aptitude test using grammaticality judgement test (GJT) as its concurrent criterion. This was done through dis-attenuated correlation, using reliability values obtained using classic test theory (CTT) and item analysis. The results show barely adequate reliability for LLAMA_F, high reliabilty values for GJT and a weak linear relationship between the two constructs. The findings of this study demonstrated that LLAMA_F might suffer from a lack of strong internal consistency. While the 200-item GJT was shown to be reliable, a few of its subcomponents were less than adequate. A further in-depth item analysis of GJT is needed in order to pare down the test and make it shorter for easier application and data collection. Finally, a nonlinear relationship between aptitude and measures of achievement is suggested for future research on language aptitude.