Crowdsourcing contest is one of the most important forms of crowdsourcing and has been adopted by more and more firms for problem-solving and decision-making. Thus it is of great importance to explore what motivates the crowd to participate in crowdsourcing contest. This paper firstly differ crowdsourcing from several related concepts and identifies four fundamental dimensions to conceptualize crowdsourcing contest, in which the motivation aspect is highly related to the other three dimensions and should be examined in an integrative framework. Then the paper theoretically examines the category of motivation based on self-determination theory and synthesizes various motivation factors in crowdsourcing contest as a form of motivation spectrum. Also, motivational affordances theory is introduced to investigate various motivation sources from an internal perspective. Furthermore, the paper builds a theoretical model to illustrate the relationships between various motivations and participation effort, in which motivational affordances and task granularity are employed as the moderate constructs. Our study will theoretically contribute to the literatures and have some potential to shed light on the system design and policy making of crowdsourcing contest.