History of language teaching methodologies is characterized by variety of syllabuses equal to the number of teaching methods. Following the ups and downs in teaching methods, syllabuses have had the same destiny. In line with the humanistic trend in this arena, whole learner involvement received prime significance to the extent that many favor negotiated syllabus in language teaching and learning; however, empirical findings are not rich enough to have strong claims in this respect. To this end, this study was an attempt to explore possible corollary among application of the negotiated syllabus, development of learners' speaking ability and modification in their attitudes and motivation towards EFL learning. To do so, a sample of 54 subjects was selected through the administration of the KEY English Test (KET) and an oral interview. They, then, were randomly divided into two groups; one experimental and one control. The experimental group received the treatment based on the negotiated syllabus. The control group, however, was exposed to conventional speaking instruction as the teacher decided. To collect required data, six instruments including: the KET, oral interviews, the Attitude-Motivation Test Battery, a speaking test and a written protocol were employed. The data were triangulated from both quantitative and qualitative perspectives. The findings revealed that there was a slight difference between the mean scores of the experimental and control group in the posttests. The posteriori interview showed statistically significant difference between groups, while the posteriori speaking test indicated slight difference between the participants' mean scores. Contrary to the expectation, the participants' attitudes and motivation did not change significantly as a result of the treatment. Believing that the nature of the target traits (i.e. speaking ability and affective factors such as attitude and motivation) is not much amenable to quantitative research, the issue was triangulated from the qualitative perspective. To this end, a written protocol was also sought from the participants so that they could report their ideas on both development of speaking ability and change in their attitudes and motivation. The respective data analysis revealed roughly contradictory results. They totally claimed improvement, or at least positive impression of developing speaking ability and positive attitude and motivation towards language learning in the light of employment of the negotiated syllabus.