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Castellated beams are generally provided with hexagonal and circular openings in the web portion. However, in view of structural applications, appropriate size and shape of openings in web are always a major issue of concern. Research work carried out in optimizing sizes of castellated beam with hexagonal openings have reported that castellated beams fail mainly by local failure modes and stress concentrations at opening edges. Castellated beams with sinusoidal openings offer better performance due to its increased area for stress distribution in addition to curved edges that causes smooth stress distribution. Few researchers have studied flexural behaviour of castellated beams with sinusoidal openings however, optimization for size of such openings has not been reported so far. The paper focuses on parametric study of castellated beam with sinusoidal openings for optimization of opening size. Finite element analysis (FEA) is carried out by Abaqus software and also by Eurocode for different opening sizes and results obtained is experimentally validated. Results show that, castellated beam with sinusoidal opening of size 0.55times the overall depth of beam gives better strength.