This study investigates 62 EFL young adults' vocabulary learning rates immediately after reading ten graded readers, and their vocabulary decay rates after a three-month period. The participants were divided into two relative levels: higher (n = 31) and lower (n = 31). One hundred target words were semi-randomly selected from the reading texts and divided into three-word frequency levels (1,000, 2,000, and 3,000 plus off-list words). The levels had 31, 36, and 34 target words respectively. The target words were tested on three occasions: pre-, post- and delayed post-tests. The results show that the immediate learning rates for the higher-level group were 68%, 71% and 69% for the 1,000, 2,000 and the off-list words respectively, and were 20%, 19% and 21% for the lower-level group. The attrition rates in the three-word frequency levels were 7%, 11% and 22% in the higher-level group, and 31%, 34% and 36% in the lower level group. The results of the present study suggest that L2 learners learned any words that they encountered while reading without considering word frequency levels, and the higher learning rate of low frequency words could be attributed to the after-reading activities and the interesting stories in the graded readers.