Cosmic power is one of the numerous renewable energy sources that can use in a Photovoltaic (PV) system or Thermal. Solar collectors play a crucial role in solar thermal systems. They convert solar radiation into heat and transfer the heat to working fluids Such as water or air. The Flat-plate collectors are the numerous common type of solar collectors and typically used as a water heater or air heater. These collectors have low efficiency and low outlet temperature. Recently, many scientists have attempted to improve the efficiency and performance of flat-plate collectors via different methods. This review paper describes the results of the experimentation carried out to study and compare the performance of the modified flat plate collector having increasing riser tube diameter and reducing riser tube length with the conventional ISI marked solar liquid flat plate collector. To study, the comparative performance characteristics of a modified flat plate collector with ISI flat plate collector operated under natural circulation mode. The suggested design found to be better than the existing ISI design of the absorber plate from an efficiency point of view. The actual useful heat gain (Qu) in the suggested design understudy found to be more by 30% than that in the case of ISI collector. However, the modified flat plate collector found to operate at a relatively lower exit temperature than the conventional ISI marked collector.