Nations progress and development is measured by what they offer to handicapped from therapeutic, educational , rehabilitative and recreational services , because they, like other community members have a right to life, growth, education and training for a career to be productive in society, Mental disability problem have a great concern; because it is a problem associated with mental efficient to humans, especially that any society depends on its development and progress on mental competency of its members, and hence it is problem of a special nature and versatile, which make it a unique model, not similar with any other disability problem. Identifying Impact of physical exercise in developing some motor abilities and improving performance of some basic motor skills for mentally disabled pupils (8-12 years). Experimental approach was used for its suitability to research nature. This study was conducted on a sample of mentally disabled students in age stage (8 -12 years) with mental age ranges between (4-6) years and IQ range between (50 - 75) degrees, Pilot studies and pre-measurements in the period from 26/01/2013 to 07/02/2013, and main experiment in the period from 09/02/2013 to 04/04/2013, and post-measurements in the period from 06/04/2013 to 08 / 4/2013. Saad Zaghloul and Ikhlas mental education schools, West Educational Department, Alexandria governorate and main study conducted on intentionally selected sample from mentally disables pupils from Saad Zaghloul mental education school, totaled to (20) students, divided into two groups, one experimental, and the other control each of (10) pupils, while the pilot studies conducted on intentionally selected sample from mentally disables pupils from Ikhlas mental education school totaled to (12) pupils. Results of the research are: 1) Experimental group which followed the proposed training program of physical exercise surpassed control group, 2) Developing motor skills for mentally disabled pupils have a positive impact in improving basic motor skills (Walking - running - throwing - catching-pushing- balance), 3) Mentally disabled pupils practice building and special purpose exercises with or withut using tools and by following scientific foundations when implemented resulted in improved motor skills for pupils with mental disabilities. At last we recommend the following points: 1) Physical exercise program to be applied to mentally disabled pupils through specialized Physical Education graduates, 2) It is necessary when planning mental schools programs to develop physical abilities programs and implement it for mentally disabled students because of its positive impact in improving performance of basic motor skills.