This paper focuses on automatic fault detection of power transmission lines using drone imagery. Given the challenges of detecting power transmission line faults due to their small size, ambiguous nature, and complex backgrounds, we propose TinyDef-DETR, a novel DETR-based detection framework. TinyDef-DETR integrates an edge-enhanced ResNet backbone that enhances edge-sensitive representations, a stride-free space-to-depth module that enables detail-preserving downsampling, a cross-stage dual-domain multi-scale attention mechanism that models global and local information together, and a Focal-Wise-SIoU regression loss function that improves the localization of small, challenging targets. Experimental results on public and real-world datasets demonstrate that TinyDef-DETR effectively mitigates the limitations of existing detectors, achieving excellent detection performance and generalization ability while maintaining reasonable computational costs. Specifically, we propose TinyDef-DETR as a suitable methodology for drone-based fault detection of power transmission lines, especially in situations involving small, ambiguous targets.