In this investigation the authors presented an attentional demand explanation for the choking under pressure occurred in sport skills. For this porpouse, attentional demands in tennis serve performance was compared under low and high pressure conditions by using dual task paradigm. Twenty tennis players served as participants and performed sixty serves as primary task in low and high (competition) conditions. Secondary task was to respond an auditory stimuli that presented during tennis serve in four probe positions. The results showed that on one hand, the pressure condition deteriorated tennis serves performance and on the other hand, it caused an increment in level of attentional resources and changeed pattern of attentinal demands. The results indicated that skillful athletes allocated more attention to their performance and more focusing to some positions of the skill under pressure condition compared to normal condition. It suggested that such attention demands changes can cause choking in sport. The results provide objective and stronge support for the predictions of self-focus theories proposed for choking in sports.