The reactive sintering mechanism of Ni-Al powder mixture near NiAl stoichiometric composition was investigated. The reaction process of the powder mixture with Ni-rich composition has been found to consist of three stages; i. e., initial reaction leading to NiAl3 and Ni2Al3 formation (873-973K), liquid phase reaction induced by heat-releasing from the initial reaction (-973K) and homogenization (above 973K). The fully dense NiAl with the Ni-rich composition can be fabricated successfully by hot-pressing at 1573-1673K. In the case of stoichiometric and Al-rich composition, sintered bodies of full density can be hardly obtained by hot pressing even at 1773K, as the result of no rapid liquid phase reaction and of marked Kirkendall effect. The formation of eutectic liquid phase by self-heating in the initial stage and subsequent liquid phase reaction are essential for full densification of the Ni-Al compound system.