The purpose of this work was to examine the effect of capsule fill weight and drug/carrier blend ratio of fluorescein isothiocyanate (FITC)-Dextran used as a model drug on the aerosolization performance from Spinhaler™. Micronised FITC-Dextran was tumbled with modified α-lactose monohydrate in the ratio of 1:25 and 4:22. Factorial design experiments were carried out to test capsule fill weights of 26 and 104 mg for aerosolization using Andersen cascade impactor (ACI). Powders were characterised in terms of particle size distribution, morphology and thermal properties. Fine particle fraction based on loaded dose (FPFTotal) was increased significantly from 14.9 to 23.1% when the blend ratio was increased from 1:25 to 4:22 (fill weight of 26 mg). The Device retention of FIT-Dextran was reduced as the fill weight and/or blend ratio increased. Mass median aerodynamic diameter (MMAD) of FITC-Dextran decreased slightly but significantly (from 3.59 to 3.27μm) with the increase in blend ratio for the fill weight 26 mg. Therefore the increase in the capsule fill weight and/or drug: carrier blend ratio improves the aerosolization performance. The effect of blend ratio was however greater compared to fill weight.