In this paper, we aim to study the basic properties of Nijenhuis operators
on coalgebras and present three algebraic structures: Nijenhuis admissible
coassociative Yang-Baxter equations (cYBes for short), Nijenhuis 2-cycles
and admissible Nijenhuis systems. We also prove that new solutions of cYBes
can be obtained by Nijenhuis admissible cYBes. The equivalence between
Nijenhuis admissible cYBes and Nijenhuis 2-cycles is given. A new way to
construct Nijenhuis operators on coalgebra is provided.