Abstract. In this article we present a new C -algebraic deformation of the Lorentz group. It is obtained by means of the Rieffel deformation applied to SL.2; C/. We give a detailed description of the resulting quantum group G D .A; / in terms of generators Ǫ; Ǒ ; O ; O ı 2 A Á -the quantum counterparts of the matrix coefficients˛,ˇ, , ı of the fundamental representation of SL.2; C/. In order to construct Ǒ -the most involved of the four generators -we first define it on the quantum Borel subgroup G 0 G, then on the quantum complement of the Borel subgroup and finally we perform the gluing procedure. In order to classify representations of the C -algebra A and to analyze the action of the comultiplication on the generators Ǫ, Ǒ , O , O ı we employ the duality in the theory of locally compact quantum groups.Mathematics Subject Classification (2010). 46L89; 58B32, 22D25.