According to Douglas Gray's The Oxford Companion to Chaucer (2003), 'imagination' in the Middle English Period is not 'creative imagination,' though it is semantically connected with a creative ability in Romanticism. In Boece, Chaucer considers and classifies 'imagination,' making use of the following adjectives: 'sensible,' 'ymaginable,' and 'resonable.' Human beings perceive things by the faculty of their 'wits' or five senses. And they have 'imagination.' They are also given 'reson,' which the other animals do not possess. They can create a new image, combining old memories and reviving things which they saw in the past. Chaucer uses the notion of 'eyes of the mind,' by which even blind persons can see things. 'Imagination' is regarded as an ability higher than the five senses, because it preserves the forms of things, separating them from the materials of the real world. This paper discusses Chaucer's imagination, investigating the description of nature in Troilus and Criseyde. The description of nature plays an important part in Chaucer's works, and is always connected with the state of mind of the characters. In keeping with the development of the stories, it reflects the subtle psychological attitude of the characters. When they are in harmony with nature, they proceed favourably, according to how the situation develops. However, when they are out of harmony with nature, they are obliged to obey an uncontrollable power such as Fortune.