Mechanical fiber splices have been in the market for quite some time. In the early, those are used as instant repairing and temporary maintenance. But now millions of splices and field-assembled no-polish connectors using mechanical splice technology have been deployed in Japan, with more being installed daily in response to the demand for high bandwidth service at every residence and business. The mechanical splice is a fiber splice where mechanical fixtures and materials perform fiber alignment and connection. Optimizing splice performance and reducing splice loss result from proper fiber end preparation and alignment. We had observed strain-dependent splice loss and temperature-dependent return loss variations among some of these mechanical fiber splices. Then we initiated a field evaluation to see how reliable these improved mechanical fiber splices have become and whether they can be left in the outside plant closures indefinitely. An aerial optic route with a 24-fiber optical cable in a humid and windy area was chosen to test mechanical fiber splices. Both field and laboratory phenomena indicated that fiber strain inside mechanical fiber splice was the cause for the greater splice loss, since index matching gel properties or splice mechanical structures should affect all splices from the same manufacturer.