In this talk, I will describe roughly how to define a generating function arithmetic divisors (in Arakelov sense) on a unitary Shimura variety of type (n-1,1). I will then briefly explain why it is modular. If time permits, I will also talk briefly about its application to Gross-Zagier-Zhang type formula and to Colmez conjecture. This is an ongoing joint work with Bruinier, Howard, Kudla and Rapoport.