Animals use their vision to detect objects in their environment and thereby guide their behavior. In particular, flying insects show behavioral responses to small moving visual objects, but neural circuits underlying such visuomotor reflexes are not clearly understood. We identified a neural pathway underlying small object avoidance in flying Drosophila. We first screened various types of visual projection neurons (VPNs) and descending neurons (DNs) by unilaterally stimulating their dendritic processes using optogenetics. We found 5 types of VPNs and 2 types of DNs that caused flies to turn away from the stimulated side. To test whether these neurons are required for small object avoidance, we reversibly blocked synaptic transmission of these neurons using thermogenetics while presenting a small moving object. We found that the amplitude of wing responses decreased significantly for 1 type of VPNs (LPLC2) and 2 types of DNs (DNp03, DNp06). From the hemibrain connectome, we found that LPLC2 and DNp06 form strong synaptic connections, likely to mediate the small object avoidance. To further substantiate this, we measured calcium responses of LPLC2 neurons using genetically encoded calcium sensors and found that they indeed responded to a small moving object. Since these neurons were previously reported to respond to a looming visual pattern, we further investigated how these neurons respond to both looming and translating visual objects. We abolished the loom response of these neurons by blocking T4/T5 cells, elementary motion detector neurons known to provide major visual inputs for the looming visual pattern. Surprisingly, their calcium response to a small translating object was only mildly affected by this genetic silencing, suggesting the existence of presynaptic visual neurons other than T4/T5 cells that provide inputs to LPLC2 neurons for small moving objects. Together, our study delineates a neural circuit underlying small object avoidance in flying Drosophila.