Effective width (b_eff) has been widely used by designers, and several specifications can be selected. However, insufficient research has been conducted on the b_eff of single and twin I-beam composite girders. Thus, finite element (FE) models, calibrated by the experiment, were used to study the effects of slab thickness, web position, interaction degree (ID) of shear studs, width-to-span ratio, and their coupling effects on b_eff. Different loading types and two definitions of b_eff were elaborated. The FE results were compared with specifications in Eurocode 4 (EC4) and AASHTO LRFD bridge design specifications (AASHTO). The following results were obtained. The modeling agrees with the experiment. The critical steel height-to-slab thickness ratio is around 4. When the ratio deviates from the critical value, b_eff is reduced with the increase of slab thickness. ID has a slight influence on b_eff. b_eff changes within 10% when ID changes within 10%. The maximum b_eff can be obtained when the web spacing-to-physical width ratio is 1/2. The decrease of web spacing brings a maximum reduction of 14% on b_eff, whereas increasing web spacing results in a maximum reduction of 70% for twin I-beam. The coupled effect of web position and slab thickness is limited. The maximum difference is 25%. Recommended equations were proposed to evaluate b_eff for single I-beam and twin I-beam composite girders based on the specifications in EC4 and AASHTO, which can ensure that the negative error is within 10%.