Relations between mechanical properties and conformational contents of native silkworm silk are investigated．Three species of silkworm cocoons

bombyx mori

tussah and eri-silkworm

are chosen and the three corresponding degummed silks are obtained by a strong alkaline solution and a relative neutral solution

respectively．For the six kinds of silk fibers

the load-elongation curves and the infrared spetra of protein are measured．To transfer load into stress

the section area of silk is directly measured by SEM observation．The contents of four conformations

α-helix

β-sheet

β-turn and random coil

can be calculated by the infrared spectra．Infrared analysis shows that the β-sheet andα-helix structures may be destroyed by the strong alkaline solution during the degumming and the destroyed molecular chains transform into random coils. So the strength and elastic modulus of bombyx mori and tussah silks are reduced drastically after degumming．In contrast

there is no significant change in the mechanical properties and the conformational content of protein for the degummed eri-silkworm silk treated by relative neutral solution．The experimental results show that fractions of β-sheet and α-helix structures

especially β-sheet

contribute to the strength and rigidity of silkworm silk．There is a good correlation between β-sheet content and mechanical properties．But the correlation is not very significant for α-helix because its content is very low．The infrared analysis also shows that the contents of β-turn structures for all silks retain invariant

which indicates that β-turn structure indeed exists in native silkworm silk and its contents seem not to be influenced by the species and the degumming．