Abstract: A field trial on fertilization of thinned middle-aged plantation of Chinese fir on hilly red-yellow soil in subtropical area was conducted from 1987~1990. Tree increment of the first year didn't show any difference among treatments, bat the 2nd- and 3rd-year increment after fertilization indicated that N fertilizer increased tree growth the most significantly, P fertilizer significantly, and K unsignificantly; while the growth response to fertilizer was higher in volume and DBH than in height, and the growth response to P or K fertilizer was smaller than that to N, NP or NPK fertilizer. The net DBH and volume increment, resulting from the repeated application of 263 kg urea per ha or 263 kg urea plus 503 kg calcium magnesium per ha in 1987 and 1989, were promoted by 36%~48% and 32%~48%,and the value of input/output was 1:3.75~3.78,respectively. The effect of background diameter on the late growth responre to fertilization was also evaluated, the larger the background DBH before fertilization, the more remarkable the fertilization efficiency. Since the second thinning in winter of 1989, only N fertilizer could significantly increase the DBH growth of the trees, whose DBH was equal to or above the average background diameter. Since the background diameter of the trees had an influence on the late growth and fertilization efficiency, it should be regulated to a similar level by thinning from below before fertilization, which is helpful to the comparison and promotion of fertilization efficiency.