In a wireless sensor network (WSN), a large number of sensor nodes have to collect and forward data hop-by-hop to a sink. This paper presents an adaptive data collection (ADC) scheme with dynamic grid-length adjustments for mobile sinks in a grid-based WSN. In the proposed ADC, a mobile sink collects data along the X- and Y-axis of the grid. Due to simultaneous collections of multiple mobile sinks, traffic distribution may become uneven and network congestion could occur. The proposed ADC can relieve the traffic by (1) adaptively adjusting transmission range between two primary grid nodes (PGN); one or more temporary grid nodes (TGN) are allocated between two PGN, and (2) dynamically changing the main data collection axis along with the moving direction of a sink. Through the inserted TGN and the changes of main collection axis, data pick up is more convenient and traffic distribution becomes more even. Additionally, less power is consumed in PGN for sustaining longer life, and since TGN provides extra buffer spaces for data forwarding, packet loss ratio is reduced. For the purpose of performance evaluation, we conduct simulation on NS-2. From the simulation results, we demonstrate that, with the proposed ADC, the overall power consumption and the packet loss ratio are significantly reduced. As a result, the overall throughput of the grid-based WSN is substantially improved.