Idiom transparency refers to how speakers think the meaning of the individual words contributes to the figurative meaning of an idiom as a whole (Gibbs, Nayak, & Cutting, 1989). However, it is not clear how speakers or language learners form their assumptions about an idiom's transparency level. This study set out to discover whether there are factors that can affect EFL learners' transparency judgement. It compared four groups of students' transparency ratings of 18 number idioms. A total of 191 students participated in the study. For the first two groups, the students were asked to make transparency judgements with the meanings of the idioms provided in L2 and L1 respectively. For the third and fourth groups, the students were asked to engage in different activities before they made transparency judgements. For the third group, the students were asked to choose a number to complete the number idioms; while for the fourth group, the students had to choose a number idiom to complete the sentences. Results showed that translation did not help the students to relate the literal meanings of the idioms to their figurative meanings. However, asking students to choose an appropriate idiom to complete the sentences could significantly raise students' transparency ratings. A closer examination found that students' ratings could be related to the internal and external semantic compatibility of the idioms. The implication of the study is that translation is not always necessary in the instruction of idioms; however, guiding students to think about the relationships between words and concepts can help them connect the literal and figurative meanings of the idioms.