Severe working conditions on Thai fishing boats have received international attention as the so-called modern slavery, at least, since 2005. Using a survey in 2012 of fishermen on fishing boats in four provinces of Thailand conducted by the International Labour Organization, this paper examines the ranges of unacceptable working conditions by extending the measurement of forced labour. This paper makes two main contributions by identifying the degree of forced labour and measuring their working conditions. First, we conceptualise two situations, forced labour and possibly forced labour, with different combinations of available variables, which is an attempt to capture the multidimentional complexity in measuring forced labour. Second, we analysed the associated working conditions among the workers. We confirmed that fishing boats are places of forced labour and destinations of trafficked persons, and that the working conditions of forced and possibly forced workers are inferior to those of other workers.