For the purpose of evaluation and improvement of thermal environment in traditional vernacular houses of Nepal, the winter thermal indoor environment was measured and investigated regarding 1) the relation between indoor and outdoor climate, 2) thermal environmental indices and 3) comparison with existing studies. 1) The thermal environment of investigated houses in ground floors / middle floor are as followings, a) indoor air temp, was 2.5K (daytime), 4.7K (nighttime) higher than outdoor air temp., b) specific humidity was 0.9g/kg' (daytime), 1.2g/kg' (nighttime) higher than outdoor specific humidity, c) vertical air temp, difference was 3.7K (daytime) greater than the ASHRAE ST-55 thermal comfort standard, that is 3K, while attic rooms are similar to outdoor. The results showed that the thermal environment are better in ground floors / middle floor than in attic rooms. 2) The mean PMV was -1.2 which was lower than the ISO 7730 standard , that is ± 0.5 and the mean SET* was 20.0℃ which was lower than the ASHRAE ST-55, that is 22.2〜 25.6℃. It was clerified that residents live in lower thermal environment than standards. 3) Compared with Japanese houses in existing studies, Nepalese houses were similar in indoor and outdoor temp, differences, while they were less in vertical temp, differences.