Nitrogen release from two low rank coals during rapid pyrolysis at 1300 degreesC has been studied with a graphite-made drop tube reactor, in which a graphite filter is installed for controlling a residence time of coal particles. Nitrogen distribution depends strongly on the time. At 0 s, 60-65% of coal-N is retained in the chars, and the rest is released as tar-N, HCN, NH3, and N-2. When the time is prolonged to 120 s, N-2 yield increases dramatically and reaches 40-55%, whereas char-N decreases mainly. There is the reverse time dependence between N-2 and char-N. Demineralization with HCl washing decreases N-2 but increases char-N, and the addition of 1 wt % Ca to the demineralized coal shows almost the reverse effect on N-2 and char-N. The deconvolution results of C(002) XRD lines reveal that the proportion of crystallized carbon in every char increases with increasing time. The linear relationship between the proportion and N-2 yield exists among all of the samples used, which strongly suggests that N-2 formation from char-N occurs in the process of carbon crystallization. Such a relationship is discussed on the basis of interactions between CaO and char matrix.