This invention provides a method and apparatus for making certain measurements across a splice of a first optical fiber and a second optical fiber, then calculating the actual decibel attenuation across the splice for optical signals transmitted through the fibers during actual use. The method of this invention involves measuring the optical power P.sub.2 in the second fiber with the fiber ends separated longitudinally a certain distance, then measuring the optical power P.sub.1 in the second fiber with the fiber ends positioned close together for effecting the splice. The attenuation is calculated from the formuladB=10 log(P.sub.1 /RP.sub.2)where R is a predetermined constant for the fibers being spliced.