Abstract: Using heterogeneous xylanase treatment on xylan, xylan samples(X0-X6) with different molecular weights were prepared by controlling the time of enzymatic hydrolysis. The enzyme treaeted xylans were dispersed/dissolved by 2% or 4% NaOH solutions and then re-precipitated by ethanol to prepare the alkali treated xylans. The effects of molecular weight on the crystallization ability of xylan were studied through X-ray diffraction(XRD) analyses on the enzyme treated and alkali treated xylan samples. The results showed that the weight average molecular weight(M_w), number average molecular weight(M_n) and polydispersity index(PDI) of xylan decreased to 25 700, 5 400 and 4.73, respectively, after heterogeneous xylanase treatment for 2.5 h. Then, with the further extension of enzymatic treatment time, the M_w of xylan changed slightly, while M_n decreased further, resulting in the increase of the PDI. It was found that the structure of xylan hydrate crystals was destroyed by heterogeneous xylanase treatment and subsequent heterogeneous alkali(2% NaOH solution) treatment, leading to the weakening of XRD peaks and the crystallinity decrease of xylan. Xylans with different molecule weights obtained by xylanase treatment were completely dissolved in 4% NaOH solution to prepare homogeneous solution. After re-precipitation, the XRD analysis showed that the complete crystalline structure of xylan hydrated crystal could be formed again, with almost unchanged crystallinity compared to that of unmodified xylan, indicating that heterogeneous reduction of the molecular weight of xylan had little influence on its crystallization ability.