A heated wood sample (46) having a water content exceeding 20 percent is compressed axially in the direction of the grain or fibres of the sample in a compression mould (10). Oppositely directed compressive forces are applied not only to the end surfaces of the sample, for example by means of a hydraulic cylinder (18), but also to side surface parts of the sample as frictional forces. The sample may be arranged within a compression chamber which is at least partly defined by longitudinally overlapping side wall parts (30, 37, 47, 48), which are mutually displaceable in the longitudinal direction of the chamber, and these side wall parts may be pressed into frictional engagement with the sample (46) for example by means of an inflatable bag or an inflatable hose section (32, 34), and means may be provided for mutually displacing the side wall parts so as to apply frictional compressive forces to the wood sample. The side wall parts or friction plates may engage with longitudinally spaced sections of the sample (46), if desired. However, if a substantially uniform compression along the length of the sample desired, the side wall parts are preferably arranged in parts (30, 37 and 47, 48) of oppositely positioned wall parts each of which extends along substantially the total length of the sample. The hydraulic cylinder (18) by means of which compressive forces are applied to the end surfaces of the sample, may simultaneously displace one pair of the oppositely positioned side wall parts longitudinally in relation to the other pair of side wall parts. The longitudinally compressed wood sample may be bent and shaped much more easily than a corresponding non-compressed sample.