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Evidence on the short-term effects of size-specific particulate matter with aerodynamic diameter ≤2.5 μm (PM 2.5), ≤10 μm (PM 10), and their difference (PM C) on children's Lower Respiratory Infections (LRI) is scare. This study aimed to estimate the differential effects of three size-specific PM on hospitalizations of children aged <18 years for pneumonia and bronchitis in 18 cities of southwestern China. The city-specific association was firstly estimated using the over-dispersed generalized additive model and then combined to obtain the regional average association. Further, to evaluate the robustness of the key findings, subgroup analyses and co-pollutant models were constructed. PM-related risks of LRI differed by PM fractions and cause-specific LRI. A 10 μg/m3 increment in PM 2.5 _lag03, PM 10 _lag06, and PM C _lag06 was associated with a 0.79% (95% CI: 0.29%, 1.29%), 0.77% (95% CI: 0.13%, 1.41%), and 2.33% (95% CI: 1.23%, 3.44%) increase in children's LRI hospitalizations, respectively. After adjustment for gaseous pollutants, adverse effects of the three types of size-specific PM on pneumonia hospitalizations were stable, ranging from 0.29% (95% CI: 0.05%, 0.54%) for PM 2.5 –2.50% (95% CI: 1.38%, 3.64%) for PM C. Additionally, PM C -related risk of bronchitis hospitalizations remained stable after adjustment for gaseous pollutants. Associations of pneumonia with PM C and PM 10 in infants, bronchitis with PM 2.5 in children aged 6–17 years, pneumonia and bronchitis with PM 2.5 , PM C , and PM 10 in children aged 1–5 years were all statistical significant. Specifically, the effects of PM 2.5 on LRI hospitalizations increased by age, with the highest effect of 1.72% (95%CI: 1.01%, 2.43%) in children aged 6–17 years. Our study provided evidence for short-term effects of different PM fractions on children LRI hospitalizations in Southwestern China, which will be useful for making and promoting policies on air quality standards in order to protect children's health. • PM-related risks of children's LRI hospitalizations differed by PM fractions. • PM 2.5 - and PM 10 -related risk of LRI was comparable, around one third of PM C. • Association of pneumonia with PM C was the strongest among three size-specific PM. • In particular, PM 2.5 -related risks of LRI hospitalizations increased by age. [ABSTRACT FROM AUTHOR]