In Wireless Sensor Networks (WSNs), due to dense deployment, sensory data gathered by sensor nodes tend to exhibit high correlation and therefore redundant. Data aggregation offers a key solution to alleviate such redundancy and reduce transmission energy consumption. In this work, we propose two aggregation mechanisms based on Received Signal Strength Indicator (RSSI). Each node maintains an RSSI value with respect to a sink node. When a node generate a data packet, that packet is associated with an aggregation ID (Agg ID). The Agg ID is used to prioritize a forwarding candidate closer to sink to be an aggregator and also a relay node in the RSSI-Based Forwarding (RBF). We show via simulation the performance of the proposed mechanism in terms of packet transmissions and receptions, energy efficiency and end-to-end delay. © 2013 IEEE.