The in-plane and through-thickness (or z-direction) tensile stress-strain properties of two kinds of commercial (or machine-made) paper and paperboard were determined in a compact testing machine equipped with a non-contacting optical extensometer. Copy paper, UKP (unbleached kraft pulp)-sack paper and uncoated 7-layer paperboard were tested at about 23 ±2°C under 55±2 % relative humidity. A dumbbell or dog-bone type specimen specified in the JIS Z 2201 for sheet materials was adopted in the in-plane tensile tests, instead of a constant-width strip specimen specified in the JIS P 8113 or ASTM D828-97. Circular specimens of uncoated paperboard were used in the through-thickness (or z-direction) tensile tests. The thicknesses of paper and uncoated paperboard sheets were carefully measured with a high-precision digital micrometer under a constant pressure. The in-plane tensile properties were measured in both the machine and cross-machine directions and were found to vary greatly, depending on the specimen size, test direction and strain rate. The through-thickness tensile properties were presented only for uncoated paperboard, because of the experimental difficulties in the z-direction tensile tests on thin paper.