Abstract: A field experiment was conducted to study the effects of nitrogen (via caboratory analysis) on the activity of the key enzymes of carbon and nitrogen metabolism, phenylaprapanoid metabolism and their relative dry-matter accumulation in flue-cured tobacco before and after topping. Under zero-nitrogen treatment (N0), results showed that before topping, the activity of the key enzymes of carbon and nitrogen metabolism was low while that of phenylalanine ammonia-lyase (PAL) was relatively high. This facilitated photosynthetic carbon to apt to the accumulation of sucrose, starch and polyphenols. Accumulation intensity of nitrogenous compounds was also low. The activity of the key enzymes of carbon and nitrogen metabolism was higher under normal nitrogen treatment (N1) than those under N0. However, the intensity of nitrogen metabolism was remarkably higher. Carbon fixation in tobacco leaf preceded nicotine accumulation and pigment concentration was generally high. The activity of the key enzymes of carbon and nitrogen metabolism was high with harmonious carbon and nitrogen metabolism under normal nitrogen plus cake-fertilizer (N2). While starch accumulation was delayed, pigments contents were high. Accumulated polyphenol substances and nicotine were between those of N0 and N1 treatments. The activity of all key enzymes activities rose shortly after topping, with consistent levels of change. Whereas leaf tobacco contents of free ammonia and pigments decreased, that of nicotine increased. Nicotine content was highest under cake-fertilizer conditions (N2). The findings suggested that nitrogen application influenced harmonious degree between carbon and nitrogen metabolism, and fates of accumulated tobacco elements. Thus nitrogen application may control the direction of tobacco leaf metabolism, and furthermore change the quality and style of tobacco leaf.