We quantitatively measured the smiles of 6 children with autism spectrum disorder (ASD-C) using a wearable interface device during animal-assisted activities (AAA) for 7 months, and compared the results with 6 normal control children of the same age. The children voluntarily participated in this study. The behavior of the participants during AAA was video-recorded and coded by the medical examiner (ME). Our major new findings are 1)the development of smile related EMG signals were found in session 4 of 5 ASD-C, 2) significant statistical differences in smiles were found between two groups in session 1, 2, and 3 (Mann-Whitney U-test, p<0.01), but the average smiles of ASD-C increased through sessions and reached the level of the average smiles of control children in session 4, 3) positive social behaviors (PSB) increased as the sessions proceeded. However the absolute value of the PSB of ASD-C was very small compared with that of the control group throughout 4 successive sessions and this is statistically evidenced (Mann-Whitney U-test, p<0.01), 4) PSB increased when the smile increased in both groups. The distribution concerning with the relationship between smiles and PSB of the ASD-C consisted of two different clusters in a lower zone and in an intermediate zone and 5) the averaged accumulative percentile of negative social behaviors (NSB) of 6 ASD-C converged to zero when smiles increased through session 1 to session 4. These findings suggest that by leading the ASD-C into a social environment that may cause smiling, the children's PSB may be facilitated and their NSB may be decreased.