Building up on Munro and Derwing, the current study set out to re-examine and generalize the Functional Load (FL) principle (Brown, 1988) as a tool to identify a set of relatively crucial segmental features for successful understanding in L2 communication. In Experiment 1, 40 Japanese learners of English in English-as-Foreign-Language settings engaged in a semi-structured task (i.e. timed picture description). Their speech was assessed by native speaking raters for overall comprehensibility (ease of understanding); and then coded for the number of high vs. low FL segmental substitution errors according to the FL principle. The results showed that it was only high FL consonant substitutions (e.g. mispronunciation of /l/ as /r/ or /v/ as /b/) that negatively impacted on native listeners' comprehensibility judgments. In Experiment 2, 40 Japanese learners of English with a wide range of immersion experience in English-speaking countries participated. The results replicated the significant impact of high FL consonant substitutions as observed in Experiment 1. Taken together, this study suggests that the FL principle may greatly help teachers and students prioritize communicatively important segmental features, a crucial step towards improving L2 oral proficiency in an effective and efficient way.