Particle matter (PM) emitted from coal combustion can cause great harm to human health and the environment. The existence of hazardous trace element arsenic (As) in PM intensifies its toxicity. In this work, four particle sizes of fly ash PM (PM<1, PM1-25, PM2.5-10, PM>10) before the electrostatic precipitator (ESP) in a 600 MW coal-fired power plant were sampled by a Dekati low pressure impactor (DLPI). The coal sample, bottom ash, and ash from ESP (ESP ash) were collected simultaneously. Concentrations of total and valent As (As3+ and Ass+) in the sample were determined by the inductively coupled plasma-mass spectrometry (ICP-MS) and high performance liquid chromatography (HPLC) coupled with ICP-MS, respectively. The morphological structure and chemical components of the PM surface were characterized by scanning electron microscopy (SEM) and X-ray fluorescence spectrometry (XRF). Results show that the coal used for the power plant is low-sulfur and low chlorine bituminous coal with 3.851 mg/kg of As. Total As content in the ESP ash is 11.44 times of that in the bottom ash (only 1.637 mg/kg). Arsenic is prone to enrich in ESP ash while dissipates in bottom ash. With the fly ash particle diameter decreasing, the concentration of total As increases from 9.599 mg/kg to 20.088 mg/kg, and the corresponding relative enrichment index increases from 0.68 to 1.42. The main As form in coal-fired fly ash is Ass+, which occupies 90.98-98.63% of the total As amount. Toxic As3+ has a little higher ratio in bottom ash with a value of 21.01%. More As physical adsorption and chemisorption active sites on the smaller fly ash PM with a higher specific area contribute to an increase in the concentration of total As and Ass+ as the particle sizes decrease. Moreover, the transformation mechanism of As to PM during coal combustion is discussed.