Graphene is an important material with potential application in spintronics. Edge-modified zigzag graphene nanoribbons (ZGNR) are investigated with density functional theory. The modifications are realized by saturating the dangling edge bonds by different terminal groups, such as H, NH2, NO2, and CH3. Such modification has a significant impact on the ZGNR electronic structure. Half-metallicity is observed when ZGNR is terminated by NO2 groups at one edge and by CH3 groups on the other side. Free energy analysis suggests that edge-modification is a practical way in experiment to realize half-metallicity.