This research has the aims to describe the students' motivation in learning English as a foreign language and to prove whether motivation influences students' achievement. The researcher replicates motivational construction from Tremblay and Gardner (1995) with some adaptation to the social environment of learners. In this construction, language attitude is determined as a factor to arise motivation. Motivation is divided into motivational antecedent and motivational behavior. This research has three variables: language attitude as an independent variable; motivational antecedent and motivational behavior as a mediating variable; and students' learning achievement as a dependent variable. The subjects consist of ninety-nine participants of Lembaga Bahasa LIA Afiliasi Bandar Lampung, one of the private English courses in Bandar Lampung. Questioners are distributed for collecting the data of motivation and the English test is used for students ' achievement. The data are analyzed in two ways: the simple profile model and the causal model. The simple profile model shows that students' motivation stresses more on instrumental rather than integrative. Most of the students studying English for the reason of having a good job in the future. Seven hypotheses have to be proved to see the influence of each part of the motivational