Nasal cavity length (NCL) is important for determining analyzed nasal segment (ANS) of acoustic rhinometry (AR). AR is a technique for nasal airway assessment which nasal cross-sectional areas and nasal volume (NV) are obtained. NCL or ANS is the important parameter to determine NV measured by AR. The ANS used to calculate NV in previous literatures vary from 4 to 8 cm. However, there has not been any study regarding NCL of Asians which is probably different from that of Western countries. To measure NCL in Thai adults using nasal telescope and compared NCL between left and right sides as well as male and female as well as among age groups. Prospective study. This study was performed on patients, aged 18-95 years, who underwent nasal telescopy under local anesthesia at the Department of Otorhinolaryngology, Siriraj Hospital. Baseline characteristics (sex and age) of patients were collected. NCL (from anterior nasal spine to posterior edge of nasal septum) of both nasal cavities was measured using rigid nasal telescope of 0 degree. Mean NCL length of both nasal cavities was calculated. There were 1277 patients, with 498 (39%) male and 779 (61%) female. The mean ± standard deviation (SD) of NCL of male was 6 ± 0.6 cm whereas that of female was 5.7 ± 0.5 cm. There were no significant differences in NCL between left and right sides as well as among age groups in each gender (p > 0.05 all). However, male had significantly longer NCL compared with that of female (p < 0.001). The mean ± SD of NCL of total population was 5.9 ± 0.6 cm. The NCL of Thais was approximately 6 cm. These data are useful to determine ANS used to calculate NV when AR is performed. The length of nasal cavity (LNC) is important variable for acoustic rhinometry (AR) which is the instrument to measure nasal volume (NV). We use AR in clinical researches to diagnose and monitor the results of treatment of nose and sinus diseasesThe LNC used to calculate NV in previous studies vary from 4 to 8 cm. However, there has not been any study of LNC of Asians which is probably different from that of Western countriesWe measured LNC in 1277 Thai adults, aged 18-95 years, with 498 (39%) male and 779 (61%) female using nasal rigid endoscope and compared LNC between male and female.The average of LNC was 5.9 cm. Male had longer LNC compared with that female. The LNC of Thais was approximately 6 cm. These data are useful for AR to calculate NV.