Let D denote an infinite alphabet -- a set that consists of infinitely many symbols. A word w = a 0 b 0 a 1 b 1 ⋯ a n b n of even length over D can be viewed as a directed graph G w whose vertices are the symbols that appear in w , and the edges are ( a 0 , b 0 ), ( a 1 , b 1 ), ..., ( a n , b n ). For a positive integer m , define a language R m such that a word w = a 0 b 0 ⋯ a n b n ∈ R m if and only if there is a path in the graph G w of length ≤ m from the vertex a 0 to the vertex b n . We establish the following hierarchy theorem for pebble automata over infinite alphabet. For every positive integer k , (i) there exists a k -pebble automaton that accepts the language R 2k − 1 ; (ii) there is no k -pebble automaton that accepts the language R 2k + 1 − 2 . Using this fact, we establish the following main results in this article: (a) a strict hierarchy of the pebble automata languages based on the number of pebbles; (b) the separation of monadic second order logic from the pebble automata languages; (c) the separation of one-way deterministic register automata languages from pebble automata languages.