When memorizing mechanical materials such as words or numbers, people have shown the tendency to overestimate their future memory due to their insensitivity to memory loss. The experiments in this paper investigate whether the same bias applies to conceptual learning and, if so, how the magnitude of this bias compares to that of mechanical learning. In our experiment, participants were divided into four groups. Two groups of participants studied a word list and took free-recall tests either two minutes or one week after learning; they also made predictions of how well they would do on the test right after learning the material. The other two groups were designed the same way except that the learning material was a concept. Our results indicate that the stability bias illusions not only applied to conceptual learning but were also more significant than in mechanical learning.