A system that makes it possible to precisely and instantaneously observe a potential energy of laser trapping as a function of three-dimensional position was developed for analyzing radiation pressure acting on a single microparticle in solution. Position sensing with a quadrant photodiode and total internal reflection microscopy are applied for measuring thermal Brownian motion of the trapped particle with∼10 nmresolution, and a trapping potential profile is estimated by a thermodynamical analysis based on the Boltzmann distribution. Characteristic features of the present system are discussed. ©1997 American Institute of Physics.