An experiment was carried out to investigate a role of phonological short-term memory on vocabulary learning in Sign Language as a second language. The subjects, who have no experiences of Sign Language learning, were required to encode 24 Sign Language new words associating with their meaning (presented in Japanese words). A 2 X 2 X 3 factorial design was used: the first variable was with or without phonological concurrent task, the second was high- or low-imagery of Sign Language words (from Matsumi's list, 2000), and the third was trial numbers of paired associate learning. Two main effects were significant on imagery and trial numbers, and an interaction between imagery and trial numbers was also significant. In terms of phonological concurrent task, however, both main effect and interaction were no significant under all conditions. These results suggested that in contrast with speech language, the phonological short-term memory did not play an important role in learning Sign Language new words.