Graphs are vastly used to represent many complex data semantics in
several domains. Subgraph isomorphism checking (an NP-complete problem) is
a regular operation with this kind of data. In this paper, we propose an improvement
of Ullmann algorithm, a well-known subgraph isomorphism checker. Our new
algorithm is called Ullmann-ON^L. It utilizes a novel sorting method for query
vertices and L-levels of vertex neighborhoods (N^L) to confine the search space of
Ullmann algorithm. Our performance study shows that Ullmann-ONL outperforms
previously proposed algorithms with a wide margin.