Objective: Late-preterm delivery is known to be associated with potential adverse effects on lung development. Passive smoking may result in alterations of pulmonary function in infants born late-preterm. Impulse oscillometry (IOS) is a noninvasive, rapid, and practicable technique that can assess lung function. This study aimed to evaluate the effect of passive smoking on lung function tests in preschool children born late-preterm using IOS. Methods: The study population consisted of a total of 139 children between 3 and 7 years of age born late-preterm who were being followed-up at our outpatient unit at the time of study period. Late-preterms were subcategorized according to presence or absence of exposure to passive smoking (PS). Those with and without exposure to passive smoking were referred to as PS group (56.1%, n = 78) and non-PS group (43.9%, n = 61), respectively. Resistance (R5-R20), reactance (X5-X20), and resonant frequency were measured by impulse oscillometry (IOS) at 5-20 Hz. Results: Median R5-R20 and Z5 were significantly higher and median X10 was significantly lower in PS group compared to non-PS group (p < .05). Conclusions: This study demonstrated that passive smoking significantly increases peripheral airway resistance and seems to adversely affect lung function in children born late-preterm.