This paper reports on the two experiments which investigate a vowel harmony effect on speech production. The first experiment looks into whether there is a general facilitative effect in vowel harmonic sequences over disharmonic ones. The second experiment concerns an implicational hierarchy, or whether native speakers are also sensitive to the distinction between the actual harmonic and disharmonic sequences in their language. The results showed that there was a facilitative effect on front harmonic, but not on height harmonic, over disharmonic sequences: such facilitative effect was cross-linguistically observed (English, Spanish, and Korean). And it was shown that Korean speakers displayed a production preference for the actual Korean type of vowel harmonic sequences, even though the harmonic feature in Korean vowel harmony cannot be defined in terms of feature system in generative grammar. Both results suggest that feature sharing does not guarantee a facilitative effect on speech production. Issues of feature sharing, articulatory economy, and frequency effects are discussed with their implications for further studies.