In this study a new type of partially perforated caisson breakwater is introduced with two shallow chambers split horizontally by an impermeable plate at the mean sea level. A plexiglass laboratory model of the structure was manufactured with different frontface perforation ratios and adjustable chamber breadths, then tested under 2D non-breaking regular and irregular waves. The horizontal wave force and wave moment are evaluated as dimensionless parameters from measured pressure-time series. A control group of tests was also conducted with plain vertical face under identical waves, also the wave loads on a vertical face for the same wave properties were computed using Goda (1985) formulation. Results show that the dimensionless wave forces and dimensionless wave moments can be reduced by a ratio up to 35%-40% under regular and irregular waves and structure performance increases with decreasing wave length. The obtained results are also compared with reference to a fully perforated (Jarlan) caisson using the method given by Tabet-Aoul and Lambert (2003) and a general harmony was seen both for the magnitudes and variation of wave loads with dimensionless chamber breadth (B/L). (C) 2009 Elsevier Ltd. All rights reserved.