This paper focuses on the partial emphasis or “prominence” of parts of Japanese sentences. Four sets of 43 read sentences uttered by two speakers including various types of prominence (172 sentences in total) are analyzed. This analysis shows that in 88% of the sentences prominence is produced by enhancing F 0 and increasing power. No examples of lengthening of phoneme duration are observed in the emphasized parts of the sentences except for some special cases. One exception is lengthening accompanied by pause insertion as a mark of prominence, and another slowing total speech rate. The prosodic features of read natural speech are then used to develop rules for changing a reference sentence to produce prominence for rule-based speech synthesis. Listening test results using 10 subjects do not show any significant difference in expressibility between prominence synthesized by rule (rate of correct expression: 76.9%) and prominence in natural speech (79.9%) at the 5% level. To further improve prominence expressibility, listening tests for 10 subjects are used to clarify the conditions under which prominence expressibility becomes optimal. These tests show that the prosodic control parameters increase the expressibility of prominence by about 20%. Finally, prosodic features of spontaneous conversational speech are analyzed and compared with those of read sentence speech. Speech-rate reduction in parts where prominence is placed is more conspicuous in spontaneous conversational speech.