Among the factors influencing learners' learning, instrumental motivation, critical thinking and autonomy are thought to be of crucial importance. The present study, thus, set out to investigate relationships between instrumental motivation, critical thinking, autonomy and academic achievement of Iranian EFL learners. To this end, 100 Iranian learners majoring in English language were selected as the participants in the study. For data collection purposes, the participants filled out two questionnaires; one on instrumental motivation, adapted from Kimura, Nakata and Okumura (2001), Carreira (2004), and Takagi (2003), and factors analysed, and another on autonomy developed by Cotterall (1995; 1999). They then sat the "California Critical Thinking Skills Test" (CCTST) form B. The participants' GPAs were also requested and collected. The results of multiple correlation analyses revealed that autonomy correlated significantly highly with academic achievement, followed by instrumental motivation and critical thinking, which stood at the second and third places respectively. The results of the multiple correlation analyses also revealed that the relationships between critical thinking and autonomy, and instrumental motivation and autonomy were significant, but critical thinking and instrumental motivation did not correlate significantly. The results of multiple regression analyses revealed that among the independent variables of the study, critical thinking was a significantly stronger predictor of academic achievement, with autonomy and instrumental motivation.