The objective of this study was to construct and assess the effectiveness of instructional program on the topic of "Environment in Sansai District, Chiang Mai Province" for Mathayom Suksa 1 Students, by establishing its effectiveness at the standard 80/80 effectiveness criterion. In addition, students learning achievement scores from both pre-test and post-test were compared after the instruction had been completed. The samples in this study were 73 students in two classes of Mathayom Suksa 1, Sansai Wittayakhom School, Sansai, Chaiang Mai. They were sampled by cluster random sampling. There were two instrument used in this study. One was 50 items achievement test on the "Environment in Sansai District, Chiang Mai Province, which had a reliability value of 0.8808. The other was the exercises within each of the four programmed instruction which were as follows: 1) general physical atmosphere of Sansai District and an introduction to the environment

2) water in Sansai District

3) forest in Sansai District

and 4) garbage in Sansai District. These programmed instructions were designed for 10 periods with two additional periods designated for pre and post testing. Data were analyzed using a computer package program, SPSS/PC+ (Statistical Package for Social Sciences/Personal Computer Plus)

to find: 1. The difficulty level (P) and discrimination index (r) of each item on the test analyzed using simple formula fo 25% technique 2. The reliability of the test analyzed using the KR20 method 3. The effectiveness of the programmed instruction analyzed using the standard 80/80 effectiveness criterion, E1/E2 4. The differences between the pre-test and post-test by using t-test method. The results of the study were as follows: 1. The programmed instruction had an effectiveness level of 96.97/87.26 which is higher than the standard 80/80 effectiveness level. 2. The achievement scores of the students in post-test were significantly different from the achievement scores in pre-test at the significant level of .001