Early intensive behavioral intervention is generally effective for children with autism spectrum disorder but is associated with variability in treatment outcome and quality of treatment delivery may contribute to this. This study examined the relationship between therapist personality, attitude toward individuals with a disability, and perceived relationship between therapist and child on procedural fidelity. Discrete Trial Teaching (DTT) was provided at a preschool for children with intellectual disabilities. Seventy DTT sessions between 22 therapists and 35 children were videotaped and analyzed. Data on therapist's attitude toward individuals with a disability, therapist's personality traits, and perceived relationship between therapist and child were also collected. Procedural fidelity was high and significantly related to therapist's attitude toward individuals with a disability, therapist's openness to experience, and perceived relationship between therapist and child. Therapists with high procedural fidelity tended to have a more positive attitude toward individuals with disabilities on the cognitive dimension, a more negative attitude toward individuals with disabilities on the affect dimension, lower levels of openness to experience, and perceived the relationship between themselves and the child as less positive.