Background: Body temperature monitoring is essential during the perioperative period. However, core body temperature measurement requires invasive device that may cause complications. This study aimed to evaluate the accuracy of non-invasive Bair Hugger™ core body temperature monitoring system (BHTMS) at the wrist compared with esophageal temperature under general anesthesia. Methods: Twenty adult patients of the American Society of Anesthesiologists physical status I or II were enrolled. BHTMS sensor was applied at wrist region. After tracheal intubation, an esophageal probe was inserted. Bair Hugger™ upper body warming blankets were used. Esophageal temperature (Teso) and BHTMS at wrist (Twrist) were recorded every 10 min. Results: Total of 257 pairs of data sets were analyzed: Teso and Twrist had no statistically significant difference (P = 0.103). Median of Teso and Twrist were 36.5°C and 36.4°C. Bland-Altman analysis showed Teso – Twrist of 0.14°C ± 1.44. Subsequently, 99 pairs of 0–40 min data set were analyzed and showed significant difference at 0 and 10 min (P < 0.001) but no significant difference at 20, 30 and 40 min. Bland– Altman plot by times showed difference (Teso - Twrist) of 1.49°C ± 2.00, 0.82°C ± 1.30, 0.29°C ± 1.32, –0.03°C ± 0.84, and –0.12°C ± 0.82 at 0, 10, 20, 30 and 40 min respectively. Conclusions BHTMS at wrist area under the upper body warming blanket is a potential alternative other than esophageal temperature for monitoring body temperature after 30 min of anesthesia induction.