A good labour productivity in construction work is required to make a good profit for a construction company. Having a productive workforce, a company can save not only construction materials and time but also nonrenewable energy. The aim of this research was to investigate factors affecting labour productivity in building construction projects of Sri Lanka in workforce’s perspective. A pretested questionnaire was used to identify how these factors affect the productivity of labourers. Labourers were individually interviewed to get the answers for their view on twenty factors. Collected data was analyzed and ranked according to the importance index. Lack of material factor acquired the highest importance index which was compatible with many other researches done around the world. Weather condition, Physical disabilities (Hearing, Speech Problems), tool and equipment shortages and Inaccurate Instructions were also ranked very important by the construction workers. However, language issues had the lowest degree of importance. Supervisor absenteeism and lack of competition were also considered relatively less important by the workers.