The construction industry faces significant labor productivity problems, mostly in developing countries. It is well understood, that the construction industry deals with a variety of workforce issues daily and undergoes external and internal influences such as inflation, experience, and skill of labor, material availability in the market, and an unstable economy, indicating a growing focus on labor efficiency. This study aimed to identify and rank factors affecting construction labor efficiency in Sri Lanka. A questionnaire survey was formulated on factors found in the literature and distributed through email across construction companies throughout the country. The survey comprised results from 100 questionnaires sent, divided into 44 pre-selected factors, and 4 main groups: human/labor, management, technical and technological, and external. The relative importance index (RII) system was used to rank the factors. Surprisingly, the outcomes of the study revealed that the external factor category scored first, even though human/labor and management factors are the first two groups that come to mind when labor efficiency is the point of question. The following are the major five factors that have the highest influence on construction project labor efficiency in Sri Lanka. 1. experience and skill of labor; 2. on-site material availability 3. material availability in the market 4. political concerns; and 5. drawing error and design variation. These outcomes of the research are expected to assist project management teams in deriving an optimum amount of work from construction labor, keeping efficiency at optimum levels, and identifying early signs of declining efficiency.