Gilles Deleuze usually articulates his philosophical problems through the reading of other philosophers. Given this matter, he typically emphasizes a concept in a philosopher’s thought which might not seem crucial at the first glance. The concept of Expression, developed by Deleuze based on his reading of Spinoza in Expressionism in Philosophy: Spinoza, is one of these concepts, which is broad and multifaceted. The present paper investigates this concept considering the relationship between unity and multiplicity. So, it is the logical aspect of the concept of expression which has been discussed here. This article attempts to show how Deleuze brings the “logic of expression” into play to reject the priority of unity over multiplicity and vice versa and establish an immanent relation between them. For this purpose, first, the distinction of three elements of this logic has been described. These elements are: “what expresses itself”, “expression”, and “what is expressed”. Then it is shown how these three elements can only be considered by each other, in such a way that none of them will have priority over the other. To do so, this correlation is explained in three different ways: Firstly, every two elements of this logic can be distinguished only in respect of the third element. Secondly, since "expression" has a genetic role in the constitution of the essence of "what expresses itself", therefore "what is expressed" can only exist in "expression". Thirdly, according to the “rule of convertibility” that Deleuze takes from Spinoza, the object and the essence of the object have a reciprocal relationship, and for this reason, all three elements of the expression are correlated. The next part of the article elucidates the relationship between the concepts of unity and multiplicity by considering this correlation. Since "expression" is the multiplicity aspect of the logic of expression and "what expresses itself" is its unity and both are connected to "what is expressed", Therefore, unity and multiplicity will be mutual concepts. This interdependence of multiplicity and unity defines the concept of “immanence” according to Deleuze. Considering this logic, the last part of the article reconstructs Deleuze's interpretation of Spinoza's concepts of absolute substance and real distinction of attributes in an analytical way, to concretely reveal an example of the critical and transformative consequences of this logic. For this purpose, by assuming the logic of expression and the immanent relation of unity and multiplicity, it is shown that the understanding of the multiplicity of attributes requires the idea of ​​an absolutely infinite substance and that the idea of ​​substance requires the multiplicity of attributes that express its essence. so that in Deleuze's reading of Spinoza, neither attributes can be conceived without absolute substance, nor absolute substance can be conceived without infinite attributes.