In　this　work,　we　investigate　the　cooperative　output　regulation　problem　of　multi-agent　uncertain　nonlinear　systems　in　lower　triangular　form　with　unknown　leader.　The　cooperative　output　regulator　construction　is　done　based　on　a　nonlinear　internal　model　approach.　Specifically,　nonlinear　internal　models　are　first　constructed,　converting　the　cooperative　output　regulation　problem　into　a　cooperative　stabilization　problem　of　the　augmented　system,　and　then　an　integral　input-to-state　stable　(iISS)　based　approach　is　proposed　to　solve　the　latter　cooperative　stabilization.　It　finally　gives　rise　to　a　constructive　state　feedback　design　for　cooperative　nonlinear　output　regulation　problem.