In order to determine of physical characteristics of rice straw as an effective source of fiber in ruminants, alfalfa hay, four varieties of rice straw (Taroum Neda, Taroum Neamat, Taroum Sangi, and Asgari), and four rations that contained four varieties of rice straws were investigated. The chemical (dry matter (DM), organic matter (OM), neutral detergent fiber (NDF), nonfiber carbohydrates (NFC) and crude protein (CP)), and physical characteristics (bulk density, water holding capacity (WHC), and soluble and insoluble DM and ash of samples, kinetics of hydration and change in functional specific gravity (FSG) and feed particle size, physically effective factor (pef)) of forages and total mixed ration (TMR) were determined. Except on ether extract and ash content, the DM, OM, NDF, NFC, and CP content of four rice straw and rations were similar but there was different among alfalfa and rice straws. The rice straws had a bulk density lesser than alfalfa. However, TMR had a similar bulk density, WHC, hydration rate, insoluble DM and ash and greater than alfalfa hay. Alfalfa had lesser WHC than rice straws and there were not different in straws. The soluble DM and FSG of rice straws were similar and lesser than those of alfalfa hay. The TMR had similar physical characteristics. Alfalfa hay had greater FSG than rice straw at all incubation times. Four rice straws and four rations were similar in in degradable NDF (iNDF) and total tract NDF digestibility (TTNDFD). Results showed that regardless the system, rice straws were similar in physically effectiveness and physically more effective than alfalfa because of having greater NDF and iNDF content, geometric mean and pef than al-falfa.