<p>Root is an important tissue to absorb water and nutrients from soil in plant and root architecture is one of critical traits influencing grain yield in crop. However, the genetic basis of root architecture remains unclear. In the present study, we identified a wild rice (<i>Oryza nivara</i>) introgression line Ra33 with longer seedling root length compared with the recipient parent 9311, an <i>indica</i> variety. Observation of longitudinal sections of root showed that the meristem length of Ra33 was significantly longer than that of 9311. Using an F₂ secondary segregating population derived from a cross between introgression line Ra33 and the recipient parent 9311, we detected a major QTL for root length at early seedling stage, <i>qROL1</i>, between the molecular markers M3 and M5 on chromosome 1, and the <i>O</i>. <i>nivara</i>-derived allele at <i>qROL1</i> increased root length under the background of 9311. In addition, the near-isogenic line NIL-<i>ROL1</i> showed a significant increase in root length compared with the recipient parent 9311, further demonstrating the genetic effect of <i>qROL1</i>. And then, a total of 159 recombinant individuals were screened from 3355 F₂ individuals and the QTL <i>qROL1</i> was narrowed down to an approximate 78 kb interval between markers M4 and RM3, including 12 predicted genes. Further sequence comparison and expression analysis of the predicted genes in the fine-mapping region indicated that eight genes might be the interesting candidates of <i>qROL1</i>. The findings will provide new clues to reveal the genetic basis of root length and genetic resources for root architecture improvement in rice.</p>