In international security cooperation, regimes rely not only on formal alliances, but also on informal security arrangement—Quasi-Alliance, which is hereby defined as permanent or ad hoc informal security cooperation arrangement, based not on military treaties, but on tacit agreements between two or among more international regimes. Quasi-Alliance is characterized by six features. First, in terms of size, there exist both bilateral and multilateral Quasi-Alliances. Second, Quasi-Alliance consists of both sovereign states and non-sovereign entities. Third, the founding of Quasi-Alliance hinges on the formation of tacit agreement between and among the elites of different states. Fourth, Quasi-Alliance can be either tacit or ad hoc, or both. Fifth, the management of Quasi-Alliance relies on mutual expectation based on communiques, joint declarations, memorandums, treaties of friendship and cooperation, declarations on the press conference, domestic laws or even UN resolutions, instead of trading military treaties. Finally, the essence of Quasi-Alliance is military and security cooperation targeting at the third party, not economic, social or cultural one.There are four encouraging variables shaping Quasi-Alliance formation, among which the common threat and development interest form the core, while ideology and cultural identity act as the periphery. Likewise, due to the differences and alliance security dilemma, there exist four discouraging variables shaping Quasi-Alliance formation: divergence, entrapment, abandonment and provocation. For its essence of security cooperation, Quasi-Alliance members not only can profit from the promises of support from quasi-allies, but need to pay their share of costs as well. Hence policy-makers may have a delicate calculation between costs and rewards. The process of their calculation involves all the encouraging and discouraging variables concerned. If they think they can only maximize the rewards by self-reliance, policy-makers would rather not engage in such cooperation and may choose neutrality; if they think they can only maximize the rewards by joining in an alliance, policy-makers will ignore their divergence and obstacles framed by alliance dilemma, tending to form alliance with the states which share the common concerns; if they think they can only maximize the rewards by participating in a limited security cooperation and meanwhile keep a certain degree of independent action, policy-makers will form a Quasi-Alliance with parties concerned, and the encouraging variables and discouraging variables will compose a multi-facet balance, which is the engine driving Quasi-Alliance formation. When regimes choose neutrality, they will tend to evade threat or hide themselves from a dispute; when regimes choose alliance, they will tend to balance or bandwagon; when they choose Quasi-Alliance, they will tend to follow quasi-balancing or quasi-bandwagoning. Multi-facet balancing is a more dynamic compared with balance of power, balance of threat or balance of interest; Quasi-Alliance is also more dynamic, compared with alliance.Quasi-Alliance exhibits functions that formal alliance cannot provide, as the security cooperation in Quasi-Alliance is secret, temporary, expedient and flexible. On