Internal exposure due to inhalation of aerosols depends on the ratio of aerodynamic shape factor (χ) to aerosol mass density (ρ). Inhaled aerosol parameters may differ from the default ρ and χ values provided by the International Commission on Radiological Protection, which are adopted for the assessment of internal exposures. This paper focuses on the influences of χ/ρ on the assessment of internal exposure to Pu for reference workers. Regional deposition fractions are found to decrease with increasing χ/ρ, and larger decreases are observed with smaller activity median aerodynamic diameter aerosols, while the slow clearance fractions (fs) in the tracheobronchial region are more sensitive for larger activity median aerodynamic diameter aerosols. Results from biokinetics calculations reveal that both the time-dependent content (excretion) and cumulative activities are determined mainly for particles initially deposited in the alveolar-interstitial region, while fs affects the local cumulative activities in the tracheobronchial region. χ/ρ is proven to have different influences for aerosols with different activity median aerodynamic diameters. The default χ/ρ values can be used when activity median aerodynamic diameters are greater than 1 μm, while one should pay attention to the value of χ/ρ when activity median aerodynamic diameters are less than 1 μm, where significant influence may be anticipated.