We demonstrate carbon nanotube intermolecular p-n junctions and study the electron transport mechanisms. Thermionic emission is the main transport mechanisms under forward bias while tunneling dominates the electron transport of the reverse bias condition. A kink point appearing on the plot of In(I/V-2) versus 1/V indicates that the transport mechanism experiences a transition from direct tunneling to the Fowler-Nordheim tunneling under the reverse bias condition. In contrast, the Arrhenius plot of the I-V curve at forward biases suggests that tunneling is more important than the thermionic emission below 50K. (c) 2012 Elsevier Ltd. All rights reserved.