Mental State Language (MSL) is language that refers to individuals' inner states, including terms relating to "emotion," "desire-&-preference," "perception" and "cognition." This study explores the nature of eight Chinese educators' MSL in their interactions with infants during structured and free play. In total, 3169 language messages (one message includes a subject and a verb) were analyzed and it was revealed that MSL messages make up 7.7% of the educators' language. In conversational interactions, educators' MSL appeared more frequently in structured than in free play. The "desire-&-preference" terms were used most frequently and tended to trigger short conversational interactions (3-4 turns). Combining the analysis of communicative functions ("question," "statement," "command," "offer"), MSL of "cognition" and "perception" presented in "question" facilitated long conversational interactions (5 turns or above). These findings inform educators about how to use MSL effectively. It raises the consideration of balancing structured and free play in nursery routines that could vary in different cultures.