Supply curve under imperfect competition or monopoly is not unique.
It is because that price is simultaneously determined along with the quantity of the commodity produced unlike perfect competition; the price is not given to a firm under monopoly or monopolistic competition.
Here the firm is a price-maker for her product. Thus the firm fixes a price where it gets maximum profits. The supply of the commodity is determined by the market demand for its product. It is therefore impossible to talk of a supply curve under monopoly or monopolistic competition.
The output supplied by the producer under such monopolistic situation will depend upon the market demand conditions for his product and no unique supply curve (as well as the supply schedule) can be drawn.
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Therefore, it is quite inapplicable to the causes of imperfect competition, monopolistic competition, monopoly and oligopoly. This is because the notion of supply curve refers to the question as to how much quantities of a commodity a firm will supply at various given prices.
Under various forms of imperfect competition, an individual firm does not take the price as given and is not a mere quantity adjuster. In fact, under various forms of imperfect competition, a firm sets its own price. For a firm under imperfect competition, it is not a question of adjusting output or supply at a given price but of choosing price-output combination which maximize its profits.
Commenting on the relevance of supply curve, Prof. Baumol writes: The supply curve is, strictly speaking, a concept which is usually relevant only for the case of pure (or perfect) competition… The reason for this lies in its definition—the supply curve is designed to answer questions of the form, “How much will firm A supply if it encounters a price which is fixed at P dollars.” But such a question is most relevant to the behaviour of firms that actually deal with prices over whose determination they exercise no influence.”