Motivated by a key agreement scheme proposed by Leighton and Micali (LM-KDS), we present a key establishment mechanism for heterogeneous sensor networks which combines random key pre-distribution schemes and LM-KDS. Using LM-KDS as its building block, proposed scheme divides nodes into groups and distributes a small number of micro key servers among nodes. This scheme consists of two phases. In the first one, some secret information is sent by the TA to every node off-line, then the TA assigns public keying material to every micro key server. In the second phase, if any pair of nodes cannot establish session key using their preloaded keying material, they may get public keying material from a micro key server through an open channel. Besides, all the public keying material preloaded in micro key servers does not need to be secret. This scheme preserves properties of LM-KDS yet takes advantage of flexibility of probabilistic approaches to yield improved resilience against node capture attacks by trading off increased communication overhead and storage complexity.