Early in 1870‟s, Dulay and Burt had proposed the Affective Filter Hypothesis and explained its influence on the foreign language learning process. Later, Krashen(1982, P. 31) developed and made the hypothesis perfect. He put the theory into five central hypotheses in second language acquisition (hereafter SLA), namely, the Acquisition-learning distinction; the Natural order hypothesis; the Monitor hypothesis; the Input hypothesis and the Affective Filter Hypothesis. Krashen argued that affective filter is a kind of psychological obstacle that prevents language learners from absorbing available comprehensible input completely. He looked affective factors functioning as a filter that reduces the amount of language input the learner is able to understand. It has a close relationship with the language learner‟s input and intake. It can be said that affective factors determine the proportion of language learners‟ input and intake. The affective factors include certain emotions, such as motivation, self-confidence, anxiety, and so on in the process of acquiring a second language. These negative emotions prevent efficient processing of the language input, and on the contrary, the positive emotions promote the efficiency of the process. When language learners with high motivation, self-confidence and a low level of anxiety, they have low filters and so receive and take in plenty of input. On the other hand, learners with low motivation, little self-confidence and a high level of anxiety have high filters and therefore obtain little input. The Affective Filter Hypothesis shows that the emotional factors strongly affect the learners‟ input and how much input is converted into intake. The Affective Filter Hypothesis has significant implications for foreign language teaching. A low filter should be created and advocated for the effective language teaching. It can be guessed that learners‟ affective filters will be influenced by teachers‟ feedback. Attempts should be made to lower the affective filter and let learners feel less stressed and more confident in a comfortable learning atmosphere.