More than 3.5 million in the US are diagnosed with autism spectrum disorder (ASD) and caregivers experience stress that adversely affects their well-being. Positive thinking training (PTT) intervention can minimize that stress. However, before testing the effectiveness of PTT, its fidelity must be established. This pilot intervention trial examined fidelity of an online PTT intervention for ASD caregivers with a random assignment of 73 caregivers to either the online PTT intervention or to the control group. Quantitative data [Positive Thinking Skills Scale (PTSS)] and qualitative data (online weekly homework) were collected. The mean scores for the PTSS improved for the intervention group and decreased for the control group post intervention. Evidence for use of PTT was found in caregivers' online weekly homework. The findings provide evidence of the implementation fidelity of PTT intervention and support moving forward to test PTT effectiveness in promoting caregivers' well-being.