The objective of this study was to compare the Bitterlich mirror relascope and the Finnish parabolic caliper in upper-stem diameter measurements, in stem volume estimation and to determine which gives the best results. The study was done in Rucamanque (old growth native forest, IX region, Chile) where thirty-seven trees were randomly chosen. Diameter measurements were taken at 2, 4 and 6 meters above the ground with relascope and Finnish caliper. The “true” or control values were directly taken on the stem with a traditional caliper, using a stairs. The relascope was unbiased for all upper-stem diameters evaluated, however, the Finnish caliper tended to overestimate (–2,2%) in the 2 meters height diameter measurements. The stem volume (below 6 meters) estimated with the relascope measurements showed to be unbiased (–0,5%). On the other hand, the stem volumes calculated with the Finnish caliper measurements (below 6 meters) were biased (–2,4%), although had a higher precision. The efficiency (bias/time of measurement) of the relascope, in order to obtain the necessary variables for stem volume estimations, was significantly higher than the efficiency obtained with the Finnish caliper.