High density small aircrafts increasingly threaten flight safety to low altitude flights. Unlike the general air transport aircrafts, small aircrafts do not install with active detection for collision avoidance. Through available surveillance communication, small aircraft can be monitored in real time while appropriate separation can be applied. A Traffic Alert and Collision Avoidance System (TCAS) algorithm is introduced to offer required conflict resolution to small aircrafts in low altitude. Based on Automatic Dependent Surveillance (ADS) concept under visual flight rule (VFR), TCAS operation can be built on ground surveillance computer as well as the airborne electronic flight instrument system (EFIS). Aircraft conflict will be detected from the surveillance information. Simple traffic resolution rules can be created for collision avoidance after alerts being generated on the ground computer and on the airborne EFIS. This paper defines the traffic advisory (TA) and resolution advisory (RA), and designs simple avoidance control to small aircrafts. In this development, ultra-light aircraft flights are operated to create the actual flight data in a small airspace. Several feasible scenarios are tested in simulations to look into the TCAS performance on the ground computer and the airborne EFIS. The results offer a wider aviation safety to low altitude flights.