This paper describes a size selective micro- / nano-particle trapping in a nanoslit membrane integrated fluidic chip (Nanoslit-Chip) for trapping and concentrating target particles in a specific region of the chip for nanoparticle based assay. For size selective particle trapping, a uniform and large scale nanoslit fluid channel array was fabricated on a silicon dioxide membrane and integrated to the PDMS microchannel. A small amount of target fluorescent particles in a large volume of sample can be concentrated on the nanoslit membrane with continuous flow and successfully detected by a fluorescent microscopy. The size selective particle trapping are demonstrated by separating two different sized particles (440 nm / 1.8 μm) in the Nanoslit-Chip. The Nanoslit-Chip is applied to the sensitive detection of proteins by trapping protein induced nanoparticle agglomerates. The Nanoslit-Chip can be effectively used for trapping and concentrating a small amount of target nanoparticles in a large sample volume.