In recent decades, the demand for flax and hemp fibers and annually renewable and biodegradable sources of raw materials is increasing rapidly due to new applications of innovative materials. Cellulose is most valuable component of bast fibres with a wide application range. Relative content of crystalline cellulose influences mechanical properties of fibres. Content and structural composition of lignin determines how severe technologies could be applied to separate crystalline cellulose from matrix. The paper examines the flax and hemp fiber structure and chemical composition of the total comparative analysis, as well as determined and compared crystalline and amorphous cellulose composition of several Latvian and grown in Poland fiber flax and hemp fibers varieties. Both flax and hemp biological, physical, chemical and mechanical properties depend on various factors and their interactions: a variety of places of production and pre-culture, climate and agricultural complexes are harvested and selected initial treatment (fiber) type and parameters which creates problems for fiber with similar characteristics. Flax and hemp straw structure, layout and design of the filament are very similar. The fiber mechanical properties are directly dependent on the cellulose content of the fibers, the degree of polymerization, mikrofibrillic angle, initial treatment type and parameters. The total crystalline cellulose constituents of flax fibres are found varying in the range from 64.57 to 75.38 %, whiles in hemp fibers varying in range from 64.2 % to 70.5 %. In addition, the fiber contains hemicellulose (12.97 to 26.07 % flax, hemp, 16.99 to 23.79 %), lignin (4.78 to 7.44 % flax , hemp, 5.68 to 7.96 %) pectin (0.45 to 3.23 % flax, hemp, 1.37 to 1.64 %), fats and waxes (0.83 to 1.9 % flax, hemp, 0.52 to 0.73 %). These data show the same chemical structure diversity within one biological culture and similarities between the two different cultures.