Graphene is a promising candidate for chemical vapor sensing. We prepared graphene sheets from highly oriented pyrolytic graphite through mechanical cleavage in order to investigate their responses to NH3 and NO2 as electron donors and acceptors, respectively. We investigated how the geometric characteristics of graphene, such as length-to-width (L/w) ratio and number of layers, affect chemical sensing properties at room temperature and ambient atmosphere. In this study, the L/w ratio of an individual graphene sheet, which is related to graphene conductivity, dominated the NH3 sensing characteristics, while the number of graphene layers had no significant effect. We also studied the effects of various thermal treatments on graphene sensitivity and recovery time in an ambient atmosphere. This study confirms the effects of geometry, operation temperature and gas concentration on the NH3 and NO2 sensing performances of graphene. (C) 2012 Elsevier B. V. All rights reserved.