Power transmission (PT) projects are vital for the power sector. However, worldwide PT projects experience delay. There is an urgent need to understand the unique causes of delays in PT projects. This paper presents the first empirical study on causes of delays in PT projects via a comprehensive literature review. Based on this literature review, 63 potential delay factors are identified and divided into ten major groups. These include two new groups of delay attributes, comprising sector-specific factors (SSF) and general factors (GF), where SSF pertains solely to PT projects and GF contributes to minimizing the bias of project participants. A questionnaire survey of 311 PT stakeholders is conducted to determine the overall ranking of the delay factors using the relative importance index. The results indicate that SSF, GF, and external/unavoidable factors are the most critical groups of delay factors, with the top-ranked factors being right of way problems of transmission line (TL), frequent changes in TL routes, and accessibility to the TL tower locations. Finally, recommendations are made to help minimize time overruns in PT projects, as well as in other linear power and non-power construction projects in general.