In this talk, we will present an immersed boundary method to simulate the creeping motion of a rigid particle in a fluid described by the Stokes equations discretized thanks to a finite element strategy on unfitted meshes, called phi-FEM. This method based on the use of a levet-set function has been successfully carried out for elliptic scalar PDEs with Dirichlet boundary conditions and for Neumann boundary conditions. One of the advantages of the method is the use of standard finite element spaces and classical integration tools.The following theoretical challenges for the Stokes problem are dealt: • the discrete inf-sup stability theory has been adapted to the case of a non-standard variational formulation of the Stokes problem posed on the immersed domain. • the integration on the whole mesh cells or facets only and the incorporation of the force balance equation have been considered. The idea of the proof of the optimal convergence in the H1 norm for the velocity and L2 for pressure and some numerical illustrations will be presented.