The purposes of this research were 1) to study the efficiency of lesson plans             of Thai Language about Royal Words Unit of Mattayomsuksa 2 Students by Using the Project-Based Learning to meet the criteria of 80/80  2) to study the effectiveness index of lesson plans of Thai Language about Royal Words Unit of Mattayomsuksa 2 Students by Using the Project-Based Learning 3) To compare  the  learning  achievements  of  Mathayomsuksa  2  students  before  and  after  treatment and 4) The satisfaction of the students in learning about Royal Words Unit by using the Project-Based Learning. The sample consisted of 52 Mathayomsueksa 2 students in class 2/8  of Khonkaenwittayayon school, under secondary educational service area  25, Meuang district, Khonkaen province. The sample is obtained by cluster random sampling technique. The instruments employed to collect data were 1) Thai language lesson plans on the Royal Words Unit of Mattayomsuksa 2 Students by using the Project-Based Learning 2) a 30 - item 4 - choice achievement test which has the discriminant index from 0.28 to 0.85  and the total reliability of 0.86 and 3) A satisfaction rating 15 item 5 level which has the discriminant index from 0.32 to 0.80  and the total reliability of 0.84  The collected data were analyzed by percentage, means, standard deviation and t-test (Dependent Sample)

The results of the research revealed that : The lesson plans of Thai Language on the Royal Words Unit of Mattayomsuksa 2 Students by using the Project-Based Learning have the efficiency  were 87.35/83.64 that higher than the criteria of 80/80.

The effectiveness index equal to 0.6196 percent from 61.96. Mattayomsuksa 2 students gained the achievements scores after treatment higher than the scores  gained  before  treatment at the significant  level  of  .05 and The  students have contentment at the highest level.