Abstract: The nutrient release patterns and decomposing rates of wheat and rapeseed straws were studied in rice fields. The straws were wrapped in net nylon bags and mulched to the fields. During the decomposing process, straw samples were taken on schedule, and were observed using electron microscope for its tissue structural images of cross sections. Results reveal that the decomposing rates of the two straws are fast at the beginning, and then slowly decrease with time. During the rice growth and development season which is about 100 d, the cumulative decomposing rates are 66.18% and 55.62% for wheat straw and rape shoot, respectively. Release rates of the nutrients of the straws are different; the K release rate is considerably greater than the rates of P and N releases. Within the first 10d, about 98.92% of K is released from wheat straw, and 98.74% from rape shoot. Based on the electron microscopic observations, there is not noticeable tissue damage of the wheat straw at the first 50 d, and after 50 d, the tissue damage mainly take place at the parenchyma cells and its surrounded vascular bundles, while the cuticle and mechanic tissues seem not damaged significantly. For rape shoot, however, the mechanic tissue damage is observed within the first 10 d when the vascular bundle cambium outside of the secondary xylem, phloem fiber, epidermal parenchyma and cuticle are all broken. These results further demonstrate that K in the straw is water extractable and readily available to plant. Therefore, amount of K in the straws should be subtracted from the recommended fertilization, while rates of N and P should be maintained in the fertilization at early crop growth stages.