This study set out to examine the Taiwan college students¡¦ underlying knowledge of English relative clauses in attempt to see what the factor(s) is/are that constrain(s) the learners¡¦ language acquisition process, whether it is the universal factor, the native language, or human general problem solving skill. Three predictor hypotheses were used in this investigation: NAPH, PDH, and SOHH, which are motivated by different theoretical backgrounds. The NPAH is based on the typological markedness, the PDH is based on the notion of human limited capacity of short term memory, and SOHH the combination of NPAH and PDH, and the structural difficulty of relative clause. The data from 84 non-English major college students of NSYSU were elicited using three kinds of tasks: sentence combination test, Chinese-English translation test, and grammaticality judgment test. 12 types of English relative clauses were analyzed in this study, namely SS, SO, SIO, SOPREP, SGEN, SOCOMP, OS, OO,OIO, OOPREP,OGEN, OOCOMP. In addition, the learners¡¦ avoidance strategies were extensively analyzed to see what was actually avoided as they were engaged in the formation of English relative clauses. Implicationally, the results suggest that Universal Grammar may be still operative in the minds of the adult FL language learners. Overall, we have the following findings: 1.The learners¡¦ acquisition of relative clauses is largely constrained by the universal markedness by NPAH, except GEN, and the order between IO and OPREP. 2.The retention of pronoun is largely constrained by the linguistic universals of NPAH, but Chinese also has certain influence on the learners¡¦ choice of supplying resumptive pronoun. 3.The learners largely did experience more difficulty in center-embedded relative clauses, which matches PDH. 4.Largely, SOHH is a valid prediction of the learners¡¦ acquisition of relative clauses. 5.The learners tended to avoid relativization on the positions low on the NPAH, except GEN. 6.The learners did frequently avoid OCOMP relatives and center-embedded relative clauses.