Similarity has been regarded as a primary means by which lexical representations are organized, and hence an important determinant of processing interactions between lexical items. A central question on lexical-semantics similarity is how it influences lexical processing. There have been much fewer investigations, however, on how lexical-semantic similarity might influence novel word learning. This dissertation work aimed to fill this gap by addressing one kind of lexical-semantic similarity, similarity among the novel words that are being learned concurrently (concurrent similarity), on the learning of phonological word forms. Importantly, it aimed to use tests that eliminated the real time processing confound at test so as to provide convincing evidence on whether learning was indeed affected by similarity. The first part of the dissertation addressed the effect of concurrent referent similarity on the learning of the phonological word forms. Experiment 1 used a naming test to provide evidence on the direction of the effect. Experiment 2 and Experiment 3 used the stem completion test and the recognition from mis-pronunciation test that controlled for real time processing between conditions. Then a 4-layer Hebbian Normalized Recurrent Network was also developed to provide even more convincing evidence on whether learning was affected (the connection weights). Consistently across the three tasks and the simulation, a detrimental effect of referent similarity on the phonological word form learning was revealed. The second part of the dissertation addressed the effect of cohort similarity on the learning of the phonological word forms. The recognition from mis-pronunciation on partial words was developed to control for real time processing between conditions so as to capture the effect of learning. We examined the effect of cohort similarity at different syllable positions and found a detrimental effect at the second syllable and non-effect at the third syllable. This is consistent with the previous finding that competition among cohorts diminishes as the stimulus is received, suggesting that the effect of cohort similarity depends on the status of competition dynamics among cohorts. The theoretical and methodological implications of this study are discussed.