Objective: To investigate the ability of the cross-sectional area (CSA) of small pulmonary vessels on chest CT in distinguishing different types of pulmonary hypertension (PH) and the severity of pulmonary arterial hypertension associated with connective tissue disease (CTD-PH). Methods: This study retrospectively included 170 patients diagnosed as PH, including 60 patients with CTD-PH, 52 patients with idiopathic PAH (iPAH) (all diagnosed by right heart catheterization), and 58 patients with chronic obstructive pulmonary disease with PH (COPD-PH) diagnosed by echocardiography. We included 120 healthy controls (HCs) for comparison. The CSA ＜5mm2 and between 5-10mm2 of small pulmonary vessels as percentage of the total lung area (%CSA＜5 and %CSA5–10) were measured on chest CT. One-way ANOVA or Kruskal Wallis test was used for comparison among the four groups as appropriate, followed by post-hoc test. Receiver operating characteristic (ROC) curve was used to evaluate the performance of %CSA for differentiating light to moderate CTD-PH (CTD-LM-PH, n=25) from severe CTD-PH (CTD-S-PH, n=35) patients. The area under the curve (AUC), the sensitivity and specificity were calculated. Results: (1) The %CSA<5 of iPH and COPD-PH were significantly lower than that of HC group (P<0.001). The %CSA5-10 of COPD-PH group was significantly lower than that of HC group (P=0.038), while the %CSA5-10 of CTD-PH and iPH groups was significantly higher than that of HC group (both P<0.05). (2) Among the four PH groups, the %CSA<5 and %CSA5-10 of CTD-PH were higher than those of COPD-PH (P<0.001). The %CSA5-10 of iPH group was significantly higher than that of CTD-PH group (P=0.022), while the %CSA<5 of iPH and COPD-PH showed no significant difference (P=0.833). (3) The %CSA<5 of CTD-S-PH group was significantly lower than that of CTD-LM-PH group (P=0.004). The ROC analysis showed that the optimal cutoff value for %CSA<5 to predict CTD-S-PH was 0.804, AUC was 0.710 (95%CI, 0.573-0.847), with a sensitivity of 71.4% and a specificity of 32.0%. Conclusion: The quantitative parameters %CSA<5 and %CSA5–10 assessed by chest CT are useful for distinguishing different types of PH. In addition, the %CSA<5 can provide information for identification of CTD-PH severity.