The interaction between input frequency and constructional interference receives little attention in second language (L2) research. Two studies were conducted to test the effect of this interaction. Study 1 examined effects of both Zipfian frequency (ZF) and balanced frequency (BF) on L2 learning of English subject-extracted relative clauses (SRs). Participants learned SRs and finished a picture description task at pretest, posttest and delayed posttest. Results suggest that ZF was not superior to BF for learners to use SRs. Study 2 tested effects of ZF and BF on L2 learning of English object-extracted relative clauses (ORs). The learning and testing procedures were the same as those in Study 1, but different from Study 1 participants either with or without previous exposure to SRs learned ORs. Results show that ZF was not superior to BF for learners without previous exposure to SRs to use ORs, but more beneficial than BF for those with previous exposure to SRs to use ORs to describe pictures. These findings demonstrate that ZF was more effective than BF in inhibiting SR interference.