In watch making micro assembly, functional clearances are very tight and the assembly of a bridge on the main plate must satisfy simultaneity several clearances. The statistical combination of several clearances having a weak yield leads to an extremely weak global assembly yield. That results in numerous and expensive final improvements and reworks. Classical worst case tolerancing method is inappropriate because of the production cost and statistical tolerancing is very risky in case of a decentered production. Inertial tolerancing offers an interesting alternative to the traditional method. Whereas a tolerance is traditionally expressed as a [Min Max] interval inertial tolerancing is based on the mean square deviation to the target. This new interpretation of the conformity makes an important cultural change in the production control. This article demonstrates the efficiency of the inertial tolerancing and proposes a comparison with traditional tolerancing method in the case of watch making micro assembly. We also show how inertial tolerancing is declined in production for guarantee the production control.