The purposes of this study were:1) to compare the effects of the task-based instruction(TBI),form-and-task-based instruction(FTBI) and conventional instruction(CI) on the students’ English learning achievement and to find their effect sized;2)to compare the effects of the TBI,FTBI and CI on the students’ grammatical accuracy of written production and to find their effect sizes;3) to compare the effects of the TBI,FTBI and CI on the students’ writing ability and to find their effect sizes ;4) to investigate the relationship between the students’ grammatical accuracy and their writing ability and to find its effect size; and 5) to survey the learners’ attitude towards the TBI,FTBI and CI. The subjects were 92 first year technical students at Rajamangala’s University of technology Suvarnabhumi. The research instruments constructed by the researcher were an achievement test, three writing tests,and a set of questionnaires. They were validated by the experts before being piloted with another group of students who had similar characteristics. The data from the main study were analyzed by means of Kruskal-Wallis H test,Rank Sums test,and T-unit analysis. The findings can be summarized as follows: 1. On average, there was no significant difference among the TBI,FTBI and CI groups on the English learning achievement(p is > 0.05). 2.On average,there was no significant difference among the TBI,FTBI and CI groups on the grammatical accuracy of the written production (p > 0.05). 3.On average, there was a significant difference among the TBI,FTBI and CI groups on the writing ability in writing 3.The writing ability of the FTBI group was significantly better than that of the TBI group (p < 0.05) and the effect size was trivial ([eta] [superscript 2] < 0.2).The writing ability of the CI group was significantly better than that of the TBI group(p < 0.05) and the effect size was trivial([eta] [superscript 2] < 0.2).However, the writing ability of the FTBI group was not significantly different from that of the CI group. 4.The relationship between the grammatical accuracy and the writing ability was significant (p is less than 0.05) and moderately positive(r[subscript xy] = 0.5-0.6). The effect size was large(d < 1.2).5.The attitude of the TBI and CI groups as measured by the seven-point semantic differential scale was quite positive,[mean] = 5.12 and 5.1 accordingly, and that of the FTBI was slightly positive,[mean]= 4.98.