This study aimed to construct and find out the efficiency of using instructional  material: modularity of Thai spelling system based on the criteria 80/80, and to compare the  achievement of learning through the instructional materials before and after learning. The study  was conducted using 15 Prathomsuksa students by a purposive sampling technique from 1  classroom of SuroaPagklong 20 School, Bang Nam Prieo District, Chachoengsao Province in the  second semester of 2007 academic year. The instruments were 9 modules of the reading skills  using the modularity of Thai spelling system, the achievement test, the specialists? assessment  form. Percentage, mean, standard deviation and t-test were used to analyze the data.
                 The study revealed that the specialists? assessment of the instructional materials with  the average mean of 4.80 which was at a high level. The efficiency of the instructional materials  was of 80.89/ 84.89 higher than the standard criteria: 80/ 80.  The students? reading ability  increased after learning by using instructional material: modularity of Thai spelling system. The  students showed the post-test mean score higher than that of the pre-test at the .01 level of significance. 
