The purpose of this study was to investigate the psychometric properties of the Thai versions of the State-Trait Anxiety Inventory for Children (STAIC) and the Child Medical Fear Scale (CMFS) in Thai school age children. Subjects were 419 Thai students in grades 3–6 in three schools in Bangkok. Classrooms of students were randomly selected, and students within each classroom were divided into two groups to test the two instruments. Data were collected from students in the classrooms twice, 2 weeks apart. Content Validity Indices of the Thai STAIC and the CMFS were between 90–95%. Internal consistencies were > .80. Test–retest reliability coefficient for the Thai CMFS was .80, and coefficients for the STAIC were .62 for A-State scale and .68 for A-Trait scale. Both instruments seem to be promising anxiety/fear measures for Thai school age children; however, the Thai version of STAIC needs modification to enhance reliability and validity. © 2000 John Wiley & Sons, Inc. Res Nurs Health 23:406–414, 2000.