Halliday believes that modality is the intermediate degrees between positive and negative poles. He also says that the space between yes and no is different for propositions and proposals. In propositions, the scales of probability and usuality are referred to as modalization and in proposals the scales of obligation and inclination are referred to as modulation. In this paper modulation and its formal expressions was analyzed in a corpus of Persian advertising and press discourses. It was observed that modulation is expressed in 5 different ways in Persian: namely by A finite verbal operator Some adverbs conveying obligation or inclination Some lexical verbs The expansion of the predicate (by a passive verb or by an adj plus a relational process verb) As a separate clause apart from the main clause It was also found that metaphorical uses of modulation like nominalization can hide the source of modulation and objectify the opinions so as the reader can not disagree with it. Modulation can also make the proposals seem more polite and decrease the obligatory and imperative load of the text and so influence more on the addressee by giving him a right to decide and manipulating him

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